Merge tag 'chrome-platform-for-linus-4.13' of git://git.kernel.org/pub/scm/linux...

[linux/fpc-iii.git] / arch / x86 / mm / mpx.c

/*
 * mpx.c - Memory Protection eXtensions
 *
 * Copyright (c) 2014, Intel Corporation.
 * Qiaowei Ren <qiaowei.ren@intel.com>
 * Dave Hansen <dave.hansen@intel.com>
 */
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/mm_types.h>
#include <linux/syscalls.h>
#include <linux/sched/sysctl.h>

#include <asm/insn.h>
#include <asm/mman.h>
#include <asm/mmu_context.h>
#include <asm/mpx.h>
#include <asm/processor.h>
#include <asm/fpu/internal.h>

#define CREATE_TRACE_POINTS
#include <asm/trace/mpx.h>

static inline unsigned long mpx_bd_size_bytes(struct mm_struct *mm)
{
        if (is_64bit_mm(mm))
                return MPX_BD_SIZE_BYTES_64;
        else
                return MPX_BD_SIZE_BYTES_32;
}

static inline unsigned long mpx_bt_size_bytes(struct mm_struct *mm)
{
        if (is_64bit_mm(mm))
                return MPX_BT_SIZE_BYTES_64;
        else
                return MPX_BT_SIZE_BYTES_32;
}

/*
 * This is really a simplified "vm_mmap". it only handles MPX
 * bounds tables (the bounds directory is user-allocated).
 */
static unsigned long mpx_mmap(unsigned long len)
{
        struct mm_struct *mm = current->mm;
        unsigned long addr, populate;

        /* Only bounds table can be allocated here */
        if (len != mpx_bt_size_bytes(mm))
                return -EINVAL;

        down_write(&mm->mmap_sem);
        addr = do_mmap(NULL, 0, len, PROT_READ | PROT_WRITE,
                       MAP_ANONYMOUS | MAP_PRIVATE, VM_MPX, 0, &populate, NULL);
        up_write(&mm->mmap_sem);
        if (populate)
                mm_populate(addr, populate);

        return addr;
}

enum reg_type {
        REG_TYPE_RM = 0,
        REG_TYPE_INDEX,
        REG_TYPE_BASE,
};

static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
                          enum reg_type type)
{
        int regno = 0;

        static const int regoff[] = {
                offsetof(struct pt_regs, ax),
                offsetof(struct pt_regs, cx),
                offsetof(struct pt_regs, dx),
                offsetof(struct pt_regs, bx),
                offsetof(struct pt_regs, sp),
                offsetof(struct pt_regs, bp),
                offsetof(struct pt_regs, si),
                offsetof(struct pt_regs, di),
#ifdef CONFIG_X86_64
                offsetof(struct pt_regs, r8),
                offsetof(struct pt_regs, r9),
                offsetof(struct pt_regs, r10),
                offsetof(struct pt_regs, r11),
                offsetof(struct pt_regs, r12),
                offsetof(struct pt_regs, r13),
                offsetof(struct pt_regs, r14),
                offsetof(struct pt_regs, r15),
#endif
        };
        int nr_registers = ARRAY_SIZE(regoff);
        /*
         * Don't possibly decode a 32-bit instructions as
         * reading a 64-bit-only register.
         */
        if (IS_ENABLED(CONFIG_X86_64) && !insn->x86_64)
                nr_registers -= 8;

        switch (type) {
        case REG_TYPE_RM:
                regno = X86_MODRM_RM(insn->modrm.value);
                if (X86_REX_B(insn->rex_prefix.value))
                        regno += 8;
                break;

        case REG_TYPE_INDEX:
                regno = X86_SIB_INDEX(insn->sib.value);
                if (X86_REX_X(insn->rex_prefix.value))
                        regno += 8;
                break;

        case REG_TYPE_BASE:
                regno = X86_SIB_BASE(insn->sib.value);
                if (X86_REX_B(insn->rex_prefix.value))
                        regno += 8;
                break;

        default:
                pr_err("invalid register type");
                BUG();
                break;
        }

        if (regno >= nr_registers) {
                WARN_ONCE(1, "decoded an instruction with an invalid register");
                return -EINVAL;
        }
        return regoff[regno];
}

/*
 * return the address being referenced be instruction
 * for rm=3 returning the content of the rm reg
 * for rm!=3 calculates the address using SIB and Disp
 */
static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
{
        unsigned long addr, base, indx;
        int addr_offset, base_offset, indx_offset;
        insn_byte_t sib;

        insn_get_modrm(insn);
        insn_get_sib(insn);
        sib = insn->sib.value;

        if (X86_MODRM_MOD(insn->modrm.value) == 3) {
                addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
                if (addr_offset < 0)
                        goto out_err;
                addr = regs_get_register(regs, addr_offset);
        } else {
                if (insn->sib.nbytes) {
                        base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
                        if (base_offset < 0)
                                goto out_err;

                        indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
                        if (indx_offset < 0)
                                goto out_err;

                        base = regs_get_register(regs, base_offset);
                        indx = regs_get_register(regs, indx_offset);
                        addr = base + indx * (1 << X86_SIB_SCALE(sib));
                } else {
                        addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
                        if (addr_offset < 0)
                                goto out_err;
                        addr = regs_get_register(regs, addr_offset);
                }
                addr += insn->displacement.value;
        }
        return (void __user *)addr;
out_err:
        return (void __user *)-1;
}

static int mpx_insn_decode(struct insn *insn,
                           struct pt_regs *regs)
{
        unsigned char buf[MAX_INSN_SIZE];
        int x86_64 = !test_thread_flag(TIF_IA32);
        int not_copied;
        int nr_copied;

        not_copied = copy_from_user(buf, (void __user *)regs->ip, sizeof(buf));
        nr_copied = sizeof(buf) - not_copied;
        /*
         * The decoder _should_ fail nicely if we pass it a short buffer.
         * But, let's not depend on that implementation detail.  If we
         * did not get anything, just error out now.
         */
        if (!nr_copied)
                return -EFAULT;
        insn_init(insn, buf, nr_copied, x86_64);
        insn_get_length(insn);
        /*
         * copy_from_user() tries to get as many bytes as we could see in
         * the largest possible instruction.  If the instruction we are
         * after is shorter than that _and_ we attempt to copy from
         * something unreadable, we might get a short read.  This is OK
         * as long as the read did not stop in the middle of the
         * instruction.  Check to see if we got a partial instruction.
         */
        if (nr_copied < insn->length)
                return -EFAULT;

        insn_get_opcode(insn);
        /*
         * We only _really_ need to decode bndcl/bndcn/bndcu
         * Error out on anything else.
         */
        if (insn->opcode.bytes[0] != 0x0f)
                goto bad_opcode;
        if ((insn->opcode.bytes[1] != 0x1a) &&
            (insn->opcode.bytes[1] != 0x1b))
                goto bad_opcode;

        return 0;
bad_opcode:
        return -EINVAL;
}

/*
 * If a bounds overflow occurs then a #BR is generated. This
 * function decodes MPX instructions to get violation address
 * and set this address into extended struct siginfo.
 *
 * Note that this is not a super precise way of doing this.
 * Userspace could have, by the time we get here, written
 * anything it wants in to the instructions.  We can not
 * trust anything about it.  They might not be valid
 * instructions or might encode invalid registers, etc...
 *
 * The caller is expected to kfree() the returned siginfo_t.
 */
siginfo_t *mpx_generate_siginfo(struct pt_regs *regs)
{
        const struct mpx_bndreg_state *bndregs;
        const struct mpx_bndreg *bndreg;
        siginfo_t *info = NULL;
        struct insn insn;
        uint8_t bndregno;
        int err;

        err = mpx_insn_decode(&insn, regs);
        if (err)
                goto err_out;

        /*
         * We know at this point that we are only dealing with
         * MPX instructions.
         */
        insn_get_modrm(&insn);
        bndregno = X86_MODRM_REG(insn.modrm.value);
        if (bndregno > 3) {
                err = -EINVAL;
                goto err_out;
        }
        /* get bndregs field from current task's xsave area */
        bndregs = get_xsave_field_ptr(XFEATURE_MASK_BNDREGS);
        if (!bndregs) {
                err = -EINVAL;
                goto err_out;
        }
        /* now go select the individual register in the set of 4 */
        bndreg = &bndregs->bndreg[bndregno];

        info = kzalloc(sizeof(*info), GFP_KERNEL);
        if (!info) {
                err = -ENOMEM;
                goto err_out;
        }
        /*
         * The registers are always 64-bit, but the upper 32
         * bits are ignored in 32-bit mode.  Also, note that the
         * upper bounds are architecturally represented in 1's
         * complement form.
         *
         * The 'unsigned long' cast is because the compiler
         * complains when casting from integers to different-size
         * pointers.
         */
        info->si_lower = (void __user *)(unsigned long)bndreg->lower_bound;
        info->si_upper = (void __user *)(unsigned long)~bndreg->upper_bound;
        info->si_addr_lsb = 0;
        info->si_signo = SIGSEGV;
        info->si_errno = 0;
        info->si_code = SEGV_BNDERR;
        info->si_addr = mpx_get_addr_ref(&insn, regs);
        /*
         * We were not able to extract an address from the instruction,
         * probably because there was something invalid in it.
         */
        if (info->si_addr == (void __user *)-1) {
                err = -EINVAL;
                goto err_out;
        }
        trace_mpx_bounds_register_exception(info->si_addr, bndreg);
        return info;
err_out:
        /* info might be NULL, but kfree() handles that */
        kfree(info);
        return ERR_PTR(err);
}

static __user void *mpx_get_bounds_dir(void)
{
        const struct mpx_bndcsr *bndcsr;

        if (!cpu_feature_enabled(X86_FEATURE_MPX))
                return MPX_INVALID_BOUNDS_DIR;

        /*
         * The bounds directory pointer is stored in a register
         * only accessible if we first do an xsave.
         */
        bndcsr = get_xsave_field_ptr(XFEATURE_MASK_BNDCSR);
        if (!bndcsr)
                return MPX_INVALID_BOUNDS_DIR;

        /*
         * Make sure the register looks valid by checking the
         * enable bit.
         */
        if (!(bndcsr->bndcfgu & MPX_BNDCFG_ENABLE_FLAG))
                return MPX_INVALID_BOUNDS_DIR;

        /*
         * Lastly, mask off the low bits used for configuration
         * flags, and return the address of the bounds table.
         */
        return (void __user *)(unsigned long)
                (bndcsr->bndcfgu & MPX_BNDCFG_ADDR_MASK);
}

int mpx_enable_management(void)
{
        void __user *bd_base = MPX_INVALID_BOUNDS_DIR;
        struct mm_struct *mm = current->mm;
        int ret = 0;

        /*
         * runtime in the userspace will be responsible for allocation of
         * the bounds directory. Then, it will save the base of the bounds
         * directory into XSAVE/XRSTOR Save Area and enable MPX through
         * XRSTOR instruction.
         *
         * The copy_xregs_to_kernel() beneath get_xsave_field_ptr() is
         * expected to be relatively expensive. Storing the bounds
         * directory here means that we do not have to do xsave in the
         * unmap path; we can just use mm->context.bd_addr instead.
         */
        bd_base = mpx_get_bounds_dir();
        down_write(&mm->mmap_sem);
        mm->context.bd_addr = bd_base;
        if (mm->context.bd_addr == MPX_INVALID_BOUNDS_DIR)
                ret = -ENXIO;

        up_write(&mm->mmap_sem);
        return ret;
}

int mpx_disable_management(void)
{
        struct mm_struct *mm = current->mm;

        if (!cpu_feature_enabled(X86_FEATURE_MPX))
                return -ENXIO;

        down_write(&mm->mmap_sem);
        mm->context.bd_addr = MPX_INVALID_BOUNDS_DIR;
        up_write(&mm->mmap_sem);
        return 0;
}

static int mpx_cmpxchg_bd_entry(struct mm_struct *mm,
                unsigned long *curval,
                unsigned long __user *addr,
                unsigned long old_val, unsigned long new_val)
{
        int ret;
        /*
         * user_atomic_cmpxchg_inatomic() actually uses sizeof()
         * the pointer that we pass to it to figure out how much
         * data to cmpxchg.  We have to be careful here not to
         * pass a pointer to a 64-bit data type when we only want
         * a 32-bit copy.
         */
        if (is_64bit_mm(mm)) {
                ret = user_atomic_cmpxchg_inatomic(curval,
                                addr, old_val, new_val);
        } else {
                u32 uninitialized_var(curval_32);
                u32 old_val_32 = old_val;
                u32 new_val_32 = new_val;
                u32 __user *addr_32 = (u32 __user *)addr;

                ret = user_atomic_cmpxchg_inatomic(&curval_32,
                                addr_32, old_val_32, new_val_32);
                *curval = curval_32;
        }
        return ret;
}

/*
 * With 32-bit mode, a bounds directory is 4MB, and the size of each
 * bounds table is 16KB. With 64-bit mode, a bounds directory is 2GB,
 * and the size of each bounds table is 4MB.
 */
static int allocate_bt(struct mm_struct *mm, long __user *bd_entry)
{
        unsigned long expected_old_val = 0;
        unsigned long actual_old_val = 0;
        unsigned long bt_addr;
        unsigned long bd_new_entry;
        int ret = 0;

        /*
         * Carve the virtual space out of userspace for the new
         * bounds table:
         */
        bt_addr = mpx_mmap(mpx_bt_size_bytes(mm));
        if (IS_ERR((void *)bt_addr))
                return PTR_ERR((void *)bt_addr);
        /*
         * Set the valid flag (kinda like _PAGE_PRESENT in a pte)
         */
        bd_new_entry = bt_addr | MPX_BD_ENTRY_VALID_FLAG;

        /*
         * Go poke the address of the new bounds table in to the
         * bounds directory entry out in userspace memory.  Note:
         * we may race with another CPU instantiating the same table.
         * In that case the cmpxchg will see an unexpected
         * 'actual_old_val'.
         *
         * This can fault, but that's OK because we do not hold
         * mmap_sem at this point, unlike some of the other part
         * of the MPX code that have to pagefault_disable().
         */
        ret = mpx_cmpxchg_bd_entry(mm, &actual_old_val, bd_entry,
                                   expected_old_val, bd_new_entry);
        if (ret)
                goto out_unmap;

        /*
         * The user_atomic_cmpxchg_inatomic() will only return nonzero
         * for faults, *not* if the cmpxchg itself fails.  Now we must
         * verify that the cmpxchg itself completed successfully.
         */
        /*
         * We expected an empty 'expected_old_val', but instead found
         * an apparently valid entry.  Assume we raced with another
         * thread to instantiate this table and desclare succecss.
         */
        if (actual_old_val & MPX_BD_ENTRY_VALID_FLAG) {
                ret = 0;
                goto out_unmap;
        }
        /*
         * We found a non-empty bd_entry but it did not have the
         * VALID_FLAG set.  Return an error which will result in
         * a SEGV since this probably means that somebody scribbled
         * some invalid data in to a bounds table.
         */
        if (expected_old_val != actual_old_val) {
                ret = -EINVAL;
                goto out_unmap;
        }
        trace_mpx_new_bounds_table(bt_addr);
        return 0;
out_unmap:
        vm_munmap(bt_addr, mpx_bt_size_bytes(mm));
        return ret;
}

/*
 * When a BNDSTX instruction attempts to save bounds to a bounds
 * table, it will first attempt to look up the table in the
 * first-level bounds directory.  If it does not find a table in
 * the directory, a #BR is generated and we get here in order to
 * allocate a new table.
 *
 * With 32-bit mode, the size of BD is 4MB, and the size of each
 * bound table is 16KB. With 64-bit mode, the size of BD is 2GB,
 * and the size of each bound table is 4MB.
 */
static int do_mpx_bt_fault(void)
{
        unsigned long bd_entry, bd_base;
        const struct mpx_bndcsr *bndcsr;
        struct mm_struct *mm = current->mm;

        bndcsr = get_xsave_field_ptr(XFEATURE_MASK_BNDCSR);
        if (!bndcsr)
                return -EINVAL;
        /*
         * Mask off the preserve and enable bits
         */
        bd_base = bndcsr->bndcfgu & MPX_BNDCFG_ADDR_MASK;
        /*
         * The hardware provides the address of the missing or invalid
         * entry via BNDSTATUS, so we don't have to go look it up.
         */
        bd_entry = bndcsr->bndstatus & MPX_BNDSTA_ADDR_MASK;
        /*
         * Make sure the directory entry is within where we think
         * the directory is.
         */
        if ((bd_entry < bd_base) ||
            (bd_entry >= bd_base + mpx_bd_size_bytes(mm)))
                return -EINVAL;

        return allocate_bt(mm, (long __user *)bd_entry);
}

int mpx_handle_bd_fault(void)
{
        /*
         * Userspace never asked us to manage the bounds tables,
         * so refuse to help.
         */
        if (!kernel_managing_mpx_tables(current->mm))
                return -EINVAL;

        return do_mpx_bt_fault();
}

/*
 * A thin wrapper around get_user_pages().  Returns 0 if the
 * fault was resolved or -errno if not.
 */
static int mpx_resolve_fault(long __user *addr, int write)
{
        long gup_ret;
        int nr_pages = 1;

        gup_ret = get_user_pages((unsigned long)addr, nr_pages,
                        write ? FOLL_WRITE : 0, NULL, NULL);
        /*
         * get_user_pages() returns number of pages gotten.
         * 0 means we failed to fault in and get anything,
         * probably because 'addr' is bad.
         */
        if (!gup_ret)
                return -EFAULT;
        /* Other error, return it */
        if (gup_ret < 0)
                return gup_ret;
        /* must have gup'd a page and gup_ret>0, success */
        return 0;
}

static unsigned long mpx_bd_entry_to_bt_addr(struct mm_struct *mm,
                                             unsigned long bd_entry)
{
        unsigned long bt_addr = bd_entry;
        int align_to_bytes;
        /*
         * Bit 0 in a bt_entry is always the valid bit.
         */
        bt_addr &= ~MPX_BD_ENTRY_VALID_FLAG;
        /*
         * Tables are naturally aligned at 8-byte boundaries
         * on 64-bit and 4-byte boundaries on 32-bit.  The
         * documentation makes it appear that the low bits
         * are ignored by the hardware, so we do the same.
         */
        if (is_64bit_mm(mm))
                align_to_bytes = 8;
        else
                align_to_bytes = 4;
        bt_addr &= ~(align_to_bytes-1);
        return bt_addr;
}

/*
 * We only want to do a 4-byte get_user() on 32-bit.  Otherwise,
 * we might run off the end of the bounds table if we are on
 * a 64-bit kernel and try to get 8 bytes.
 */
static int get_user_bd_entry(struct mm_struct *mm, unsigned long *bd_entry_ret,
                long __user *bd_entry_ptr)
{
        u32 bd_entry_32;
        int ret;

        if (is_64bit_mm(mm))
                return get_user(*bd_entry_ret, bd_entry_ptr);

        /*
         * Note that get_user() uses the type of the *pointer* to
         * establish the size of the get, not the destination.
         */
        ret = get_user(bd_entry_32, (u32 __user *)bd_entry_ptr);
        *bd_entry_ret = bd_entry_32;
        return ret;
}

/*
 * Get the base of bounds tables pointed by specific bounds
 * directory entry.
 */
static int get_bt_addr(struct mm_struct *mm,
                        long __user *bd_entry_ptr,
                        unsigned long *bt_addr_result)
{
        int ret;
        int valid_bit;
        unsigned long bd_entry;
        unsigned long bt_addr;

        if (!access_ok(VERIFY_READ, (bd_entry_ptr), sizeof(*bd_entry_ptr)))
                return -EFAULT;

        while (1) {
                int need_write = 0;

                pagefault_disable();
                ret = get_user_bd_entry(mm, &bd_entry, bd_entry_ptr);
                pagefault_enable();
                if (!ret)
                        break;
                if (ret == -EFAULT)
                        ret = mpx_resolve_fault(bd_entry_ptr, need_write);
                /*
                 * If we could not resolve the fault, consider it
                 * userspace's fault and error out.
                 */
                if (ret)
                        return ret;
        }

        valid_bit = bd_entry & MPX_BD_ENTRY_VALID_FLAG;
        bt_addr = mpx_bd_entry_to_bt_addr(mm, bd_entry);

        /*
         * When the kernel is managing bounds tables, a bounds directory
         * entry will either have a valid address (plus the valid bit)
         * *OR* be completely empty. If we see a !valid entry *and* some
         * data in the address field, we know something is wrong. This
         * -EINVAL return will cause a SIGSEGV.
         */
        if (!valid_bit && bt_addr)
                return -EINVAL;
        /*
         * Do we have an completely zeroed bt entry?  That is OK.  It
         * just means there was no bounds table for this memory.  Make
         * sure to distinguish this from -EINVAL, which will cause
         * a SEGV.
         */
        if (!valid_bit)
                return -ENOENT;

        *bt_addr_result = bt_addr;
        return 0;
}

static inline int bt_entry_size_bytes(struct mm_struct *mm)
{
        if (is_64bit_mm(mm))
                return MPX_BT_ENTRY_BYTES_64;
        else
                return MPX_BT_ENTRY_BYTES_32;
}

/*
 * Take a virtual address and turns it in to the offset in bytes
 * inside of the bounds table where the bounds table entry
 * controlling 'addr' can be found.
 */
static unsigned long mpx_get_bt_entry_offset_bytes(struct mm_struct *mm,
                unsigned long addr)
{
        unsigned long bt_table_nr_entries;
        unsigned long offset = addr;

        if (is_64bit_mm(mm)) {
                /* Bottom 3 bits are ignored on 64-bit */
                offset >>= 3;
                bt_table_nr_entries = MPX_BT_NR_ENTRIES_64;
        } else {
                /* Bottom 2 bits are ignored on 32-bit */
                offset >>= 2;
                bt_table_nr_entries = MPX_BT_NR_ENTRIES_32;
        }
        /*
         * We know the size of the table in to which we are
         * indexing, and we have eliminated all the low bits
         * which are ignored for indexing.
         *
         * Mask out all the high bits which we do not need
         * to index in to the table.  Note that the tables
         * are always powers of two so this gives us a proper
         * mask.
         */
        offset &= (bt_table_nr_entries-1);
        /*
         * We now have an entry offset in terms of *entries* in
         * the table.  We need to scale it back up to bytes.
         */
        offset *= bt_entry_size_bytes(mm);
        return offset;
}

/*
 * How much virtual address space does a single bounds
 * directory entry cover?
 *
 * Note, we need a long long because 4GB doesn't fit in
 * to a long on 32-bit.
 */
static inline unsigned long bd_entry_virt_space(struct mm_struct *mm)
{
        unsigned long long virt_space;
        unsigned long long GB = (1ULL << 30);

        /*
         * This covers 32-bit emulation as well as 32-bit kernels
         * running on 64-bit hardware.
         */
        if (!is_64bit_mm(mm))
                return (4ULL * GB) / MPX_BD_NR_ENTRIES_32;

        /*
         * 'x86_virt_bits' returns what the hardware is capable
         * of, and returns the full >32-bit address space when
         * running 32-bit kernels on 64-bit hardware.
         */
        virt_space = (1ULL << boot_cpu_data.x86_virt_bits);
        return virt_space / MPX_BD_NR_ENTRIES_64;
}

/*
 * Free the backing physical pages of bounds table 'bt_addr'.
 * Assume start...end is within that bounds table.
 */
static noinline int zap_bt_entries_mapping(struct mm_struct *mm,
                unsigned long bt_addr,
                unsigned long start_mapping, unsigned long end_mapping)
{
        struct vm_area_struct *vma;
        unsigned long addr, len;
        unsigned long start;
        unsigned long end;

        /*
         * if we 'end' on a boundary, the offset will be 0 which
         * is not what we want.  Back it up a byte to get the
         * last bt entry.  Then once we have the entry itself,
         * move 'end' back up by the table entry size.
         */
        start = bt_addr + mpx_get_bt_entry_offset_bytes(mm, start_mapping);
        end   = bt_addr + mpx_get_bt_entry_offset_bytes(mm, end_mapping - 1);
        /*
         * Move end back up by one entry.  Among other things
         * this ensures that it remains page-aligned and does
         * not screw up zap_page_range()
         */
        end += bt_entry_size_bytes(mm);

        /*
         * Find the first overlapping vma. If vma->vm_start > start, there
         * will be a hole in the bounds table. This -EINVAL return will
         * cause a SIGSEGV.
         */
        vma = find_vma(mm, start);
        if (!vma || vma->vm_start > start)
                return -EINVAL;

        /*
         * A NUMA policy on a VM_MPX VMA could cause this bounds table to
         * be split. So we need to look across the entire 'start -> end'
         * range of this bounds table, find all of the VM_MPX VMAs, and
         * zap only those.
         */
        addr = start;
        while (vma && vma->vm_start < end) {
                /*
                 * We followed a bounds directory entry down
                 * here.  If we find a non-MPX VMA, that's bad,
                 * so stop immediately and return an error.  This
                 * probably results in a SIGSEGV.
                 */
                if (!(vma->vm_flags & VM_MPX))
                        return -EINVAL;

                len = min(vma->vm_end, end) - addr;
                zap_page_range(vma, addr, len);
                trace_mpx_unmap_zap(addr, addr+len);

                vma = vma->vm_next;
                addr = vma->vm_start;
        }
        return 0;
}

static unsigned long mpx_get_bd_entry_offset(struct mm_struct *mm,
                unsigned long addr)
{
        /*
         * There are several ways to derive the bd offsets.  We
         * use the following approach here:
         * 1. We know the size of the virtual address space
         * 2. We know the number of entries in a bounds table
         * 3. We know that each entry covers a fixed amount of
         *    virtual address space.
         * So, we can just divide the virtual address by the
         * virtual space used by one entry to determine which
         * entry "controls" the given virtual address.
         */
        if (is_64bit_mm(mm)) {
                int bd_entry_size = 8; /* 64-bit pointer */
                /*
                 * Take the 64-bit addressing hole in to account.
                 */
                addr &= ((1UL << boot_cpu_data.x86_virt_bits) - 1);
                return (addr / bd_entry_virt_space(mm)) * bd_entry_size;
        } else {
                int bd_entry_size = 4; /* 32-bit pointer */
                /*
                 * 32-bit has no hole so this case needs no mask
                 */
                return (addr / bd_entry_virt_space(mm)) * bd_entry_size;
        }
        /*
         * The two return calls above are exact copies.  If we
         * pull out a single copy and put it in here, gcc won't
         * realize that we're doing a power-of-2 divide and use
         * shifts.  It uses a real divide.  If we put them up
         * there, it manages to figure it out (gcc 4.8.3).
         */
}

static int unmap_entire_bt(struct mm_struct *mm,
                long __user *bd_entry, unsigned long bt_addr)
{
        unsigned long expected_old_val = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
        unsigned long uninitialized_var(actual_old_val);
        int ret;

        while (1) {
                int need_write = 1;
                unsigned long cleared_bd_entry = 0;

                pagefault_disable();
                ret = mpx_cmpxchg_bd_entry(mm, &actual_old_val,
                                bd_entry, expected_old_val, cleared_bd_entry);
                pagefault_enable();
                if (!ret)
                        break;
                if (ret == -EFAULT)
                        ret = mpx_resolve_fault(bd_entry, need_write);
                /*
                 * If we could not resolve the fault, consider it
                 * userspace's fault and error out.
                 */
                if (ret)
                        return ret;
        }
        /*
         * The cmpxchg was performed, check the results.
         */
        if (actual_old_val != expected_old_val) {
                /*
                 * Someone else raced with us to unmap the table.
                 * That is OK, since we were both trying to do
                 * the same thing.  Declare success.
                 */
                if (!actual_old_val)
                        return 0;
                /*
                 * Something messed with the bounds directory
                 * entry.  We hold mmap_sem for read or write
                 * here, so it could not be a _new_ bounds table
                 * that someone just allocated.  Something is
                 * wrong, so pass up the error and SIGSEGV.
                 */
                return -EINVAL;
        }
        /*
         * Note, we are likely being called under do_munmap() already. To
         * avoid recursion, do_munmap() will check whether it comes
         * from one bounds table through VM_MPX flag.
         */
        return do_munmap(mm, bt_addr, mpx_bt_size_bytes(mm), NULL);
}

static int try_unmap_single_bt(struct mm_struct *mm,
               unsigned long start, unsigned long end)
{
        struct vm_area_struct *next;
        struct vm_area_struct *prev;
        /*
         * "bta" == Bounds Table Area: the area controlled by the
         * bounds table that we are unmapping.
         */
        unsigned long bta_start_vaddr = start & ~(bd_entry_virt_space(mm)-1);
        unsigned long bta_end_vaddr = bta_start_vaddr + bd_entry_virt_space(mm);
        unsigned long uninitialized_var(bt_addr);
        void __user *bde_vaddr;
        int ret;
        /*
         * We already unlinked the VMAs from the mm's rbtree so 'start'
         * is guaranteed to be in a hole. This gets us the first VMA
         * before the hole in to 'prev' and the next VMA after the hole
         * in to 'next'.
         */
        next = find_vma_prev(mm, start, &prev);
        /*
         * Do not count other MPX bounds table VMAs as neighbors.
         * Although theoretically possible, we do not allow bounds
         * tables for bounds tables so our heads do not explode.
         * If we count them as neighbors here, we may end up with
         * lots of tables even though we have no actual table
         * entries in use.
         */
        while (next && (next->vm_flags & VM_MPX))
                next = next->vm_next;
        while (prev && (prev->vm_flags & VM_MPX))
                prev = prev->vm_prev;
        /*
         * We know 'start' and 'end' lie within an area controlled
         * by a single bounds table.  See if there are any other
         * VMAs controlled by that bounds table.  If there are not
         * then we can "expand" the are we are unmapping to possibly
         * cover the entire table.
         */
        next = find_vma_prev(mm, start, &prev);
        if ((!prev || prev->vm_end <= bta_start_vaddr) &&
            (!next || next->vm_start >= bta_end_vaddr)) {
                /*
                 * No neighbor VMAs controlled by same bounds
                 * table.  Try to unmap the whole thing
                 */
                start = bta_start_vaddr;
                end = bta_end_vaddr;
        }

        bde_vaddr = mm->context.bd_addr + mpx_get_bd_entry_offset(mm, start);
        ret = get_bt_addr(mm, bde_vaddr, &bt_addr);
        /*
         * No bounds table there, so nothing to unmap.
         */
        if (ret == -ENOENT) {
                ret = 0;
                return 0;
        }
        if (ret)
                return ret;
        /*
         * We are unmapping an entire table.  Either because the
         * unmap that started this whole process was large enough
         * to cover an entire table, or that the unmap was small
         * but was the area covered by a bounds table.
         */
        if ((start == bta_start_vaddr) &&
            (end == bta_end_vaddr))
                return unmap_entire_bt(mm, bde_vaddr, bt_addr);
        return zap_bt_entries_mapping(mm, bt_addr, start, end);
}

static int mpx_unmap_tables(struct mm_struct *mm,
                unsigned long start, unsigned long end)
{
        unsigned long one_unmap_start;
        trace_mpx_unmap_search(start, end);

        one_unmap_start = start;
        while (one_unmap_start < end) {
                int ret;
                unsigned long next_unmap_start = ALIGN(one_unmap_start+1,
                                                       bd_entry_virt_space(mm));
                unsigned long one_unmap_end = end;
                /*
                 * if the end is beyond the current bounds table,
                 * move it back so we only deal with a single one
                 * at a time
                 */
                if (one_unmap_end > next_unmap_start)
                        one_unmap_end = next_unmap_start;
                ret = try_unmap_single_bt(mm, one_unmap_start, one_unmap_end);
                if (ret)
                        return ret;

                one_unmap_start = next_unmap_start;
        }
        return 0;
}

/*
 * Free unused bounds tables covered in a virtual address region being
 * munmap()ed. Assume end > start.
 *
 * This function will be called by do_munmap(), and the VMAs covering
 * the virtual address region start...end have already been split if
 * necessary, and the 'vma' is the first vma in this range (start -> end).
 */
void mpx_notify_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
                unsigned long start, unsigned long end)
{
        int ret;

        /*
         * Refuse to do anything unless userspace has asked
         * the kernel to help manage the bounds tables,
         */
        if (!kernel_managing_mpx_tables(current->mm))
                return;
        /*
         * This will look across the entire 'start -> end' range,
         * and find all of the non-VM_MPX VMAs.
         *
         * To avoid recursion, if a VM_MPX vma is found in the range
         * (start->end), we will not continue follow-up work. This
         * recursion represents having bounds tables for bounds tables,
         * which should not occur normally. Being strict about it here
         * helps ensure that we do not have an exploitable stack overflow.
         */
        do {
                if (vma->vm_flags & VM_MPX)
                        return;
                vma = vma->vm_next;
        } while (vma && vma->vm_start < end);

        ret = mpx_unmap_tables(mm, start, end);
        if (ret)
                force_sig(SIGSEGV, current);
}