arch/x86/kernel/tls.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/kernel.h>
   3 #include <linux/errno.h>
   4 #include <linux/sched.h>
   5 #include <linux/user.h>
   6 #include <linux/regset.h>
   7 #include <linux/syscalls.h>
   8 #include <linux/nospec.h>
   9
  10 #include <linux/uaccess.h>
  11 #include <asm/desc.h>
  12 #include <asm/ldt.h>
  13 #include <asm/processor.h>
  14 #include <asm/proto.h>
  15 #include <asm/gsseg.h>
  16
  17 #include "tls.h"
  18
  19 /*
  20  * sys_alloc_thread_area: get a yet unused TLS descriptor index.
  21  */
  22 static int get_free_idx(void)
  23 {
  24         struct thread_struct *t = &current->thread;
  25         int idx;
  26
  27         for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
  28                 if (desc_empty(&t->tls_array[idx]))
  29                         return idx + GDT_ENTRY_TLS_MIN;
  30         return -ESRCH;
  31 }
  32
  33 static bool tls_desc_okay(const struct user_desc *info)
  34 {
  35         /*
  36          * For historical reasons (i.e. no one ever documented how any
  37          * of the segmentation APIs work), user programs can and do
  38          * assume that a struct user_desc that's all zeros except for
  39          * entry_number means "no segment at all".  This never actually
  40          * worked.  In fact, up to Linux 3.19, a struct user_desc like
  41          * this would create a 16-bit read-write segment with base and
  42          * limit both equal to zero.
  43          *
  44          * That was close enough to "no segment at all" until we
  45          * hardened this function to disallow 16-bit TLS segments.  Fix
  46          * it up by interpreting these zeroed segments the way that they
  47          * were almost certainly intended to be interpreted.
  48          *
  49          * The correct way to ask for "no segment at all" is to specify
  50          * a user_desc that satisfies LDT_empty.  To keep everything
  51          * working, we accept both.
  52          *
  53          * Note that there's a similar kludge in modify_ldt -- look at
  54          * the distinction between modes 1 and 0x11.
  55          */
  56         if (LDT_empty(info) || LDT_zero(info))
  57                 return true;
  58
  59         /*
  60          * espfix is required for 16-bit data segments, but espfix
  61          * only works for LDT segments.
  62          */
  63         if (!info->seg_32bit)
  64                 return false;
  65
  66         /* Only allow data segments in the TLS array. */
  67         if (info->contents > 1)
  68                 return false;
  69
  70         /*
  71          * Non-present segments with DPL 3 present an interesting attack
  72          * surface.  The kernel should handle such segments correctly,
  73          * but TLS is very difficult to protect in a sandbox, so prevent
  74          * such segments from being created.
  75          *
  76          * If userspace needs to remove a TLS entry, it can still delete
  77          * it outright.
  78          */
  79         if (info->seg_not_present)
  80                 return false;
  81
  82         return true;
  83 }
  84
  85 static void set_tls_desc(struct task_struct *p, int idx,
  86                          const struct user_desc *info, int n)
  87 {
  88         struct thread_struct *t = &p->thread;
  89         struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
  90         int cpu;
  91
  92         /*
  93          * We must not get preempted while modifying the TLS.
  94          */
  95         cpu = get_cpu();
  96
  97         while (n-- > 0) {
  98                 if (LDT_empty(info) || LDT_zero(info))
  99                         memset(desc, 0, sizeof(*desc));
 100                 else
 101                         fill_ldt(desc, info);
 102                 ++info;
 103                 ++desc;
 104         }
 105
 106         if (t == &current->thread)
 107                 load_TLS(t, cpu);
 108
 109         put_cpu();
 110 }
 111
 112 /*
 113  * Set a given TLS descriptor:
 114  */
 115 int do_set_thread_area(struct task_struct *p, int idx,
 116                        struct user_desc __user *u_info,
 117                        int can_allocate)
 118 {
 119         struct user_desc info;
 120         unsigned short __maybe_unused sel, modified_sel;
 121
 122         if (copy_from_user(&info, u_info, sizeof(info)))
 123                 return -EFAULT;
 124
 125         if (!tls_desc_okay(&info))
 126                 return -EINVAL;
 127
 128         if (idx == -1)
 129                 idx = info.entry_number;
 130
 131         /*
 132          * index -1 means the kernel should try to find and
 133          * allocate an empty descriptor:
 134          */
 135         if (idx == -1 && can_allocate) {
 136                 idx = get_free_idx();
 137                 if (idx < 0)
 138                         return idx;
 139                 if (put_user(idx, &u_info->entry_number))
 140                         return -EFAULT;
 141         }
 142
 143         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 144                 return -EINVAL;
 145
 146         set_tls_desc(p, idx, &info, 1);
 147
 148         /*
 149          * If DS, ES, FS, or GS points to the modified segment, forcibly
 150          * refresh it.  Only needed on x86_64 because x86_32 reloads them
 151          * on return to user mode.
 152          */
 153         modified_sel = (idx << 3) | 3;
 154
 155         if (p == current) {
 156 #ifdef CONFIG_X86_64
 157                 savesegment(ds, sel);
 158                 if (sel == modified_sel)
 159                         loadsegment(ds, sel);
 160
 161                 savesegment(es, sel);
 162                 if (sel == modified_sel)
 163                         loadsegment(es, sel);
 164
 165                 savesegment(fs, sel);
 166                 if (sel == modified_sel)
 167                         loadsegment(fs, sel);
 168 #endif
 169
 170                 savesegment(gs, sel);
 171                 if (sel == modified_sel)
 172                         load_gs_index(sel);
 173         } else {
 174 #ifdef CONFIG_X86_64
 175                 if (p->thread.fsindex == modified_sel)
 176                         p->thread.fsbase = info.base_addr;
 177
 178                 if (p->thread.gsindex == modified_sel)
 179                         p->thread.gsbase = info.base_addr;
 180 #endif
 181         }
 182
 183         return 0;
 184 }
 185
 186 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 187 {
 188         return do_set_thread_area(current, -1, u_info, 1);
 189 }
 190
 191
 192 /*
 193  * Get the current Thread-Local Storage area:
 194  */
 195
 196 static void fill_user_desc(struct user_desc *info, int idx,
 197                            const struct desc_struct *desc)
 198
 199 {
 200         memset(info, 0, sizeof(*info));
 201         info->entry_number = idx;
 202         info->base_addr = get_desc_base(desc);
 203         info->limit = get_desc_limit(desc);
 204         info->seg_32bit = desc->d;
 205         info->contents = desc->type >> 2;
 206         info->read_exec_only = !(desc->type & 2);
 207         info->limit_in_pages = desc->g;
 208         info->seg_not_present = !desc->p;
 209         info->useable = desc->avl;
 210 #ifdef CONFIG_X86_64
 211         info->lm = desc->l;
 212 #endif
 213 }
 214
 215 int do_get_thread_area(struct task_struct *p, int idx,
 216                        struct user_desc __user *u_info)
 217 {
 218         struct user_desc info;
 219         int index;
 220
 221         if (idx == -1 && get_user(idx, &u_info->entry_number))
 222                 return -EFAULT;
 223
 224         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 225                 return -EINVAL;
 226
 227         index = idx - GDT_ENTRY_TLS_MIN;
 228         index = array_index_nospec(index,
 229                         GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);
 230
 231         fill_user_desc(&info, idx, &p->thread.tls_array[index]);
 232
 233         if (copy_to_user(u_info, &info, sizeof(info)))
 234                 return -EFAULT;
 235         return 0;
 236 }
 237
 238 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 239 {
 240         return do_get_thread_area(current, -1, u_info);
 241 }
 242
 243 int regset_tls_active(struct task_struct *target,
 244                       const struct user_regset *regset)
 245 {
 246         struct thread_struct *t = &target->thread;
 247         int n = GDT_ENTRY_TLS_ENTRIES;
 248         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 249                 --n;
 250         return n;
 251 }
 252
 253 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 254                    struct membuf to)
 255 {
 256         const struct desc_struct *tls;
 257         struct user_desc v;
 258         int pos;
 259
 260         for (pos = 0, tls = target->thread.tls_array; to.left; pos++, tls++) {
 261                 fill_user_desc(&v, GDT_ENTRY_TLS_MIN + pos, tls);
 262                 membuf_write(&to, &v, sizeof(v));
 263         }
 264         return 0;
 265 }
 266
 267 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 268                    unsigned int pos, unsigned int count,
 269                    const void *kbuf, const void __user *ubuf)
 270 {
 271         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 272         const struct user_desc *info;
 273         int i;
 274
 275         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 276             (pos % sizeof(struct user_desc)) != 0 ||
 277             (count % sizeof(struct user_desc)) != 0)
 278                 return -EINVAL;
 279
 280         if (kbuf)
 281                 info = kbuf;
 282         else if (__copy_from_user(infobuf, ubuf, count))
 283                 return -EFAULT;
 284         else
 285                 info = infobuf;
 286
 287         for (i = 0; i < count / sizeof(struct user_desc); i++)
 288                 if (!tls_desc_okay(info + i))
 289                         return -EINVAL;
 290
 291         set_tls_desc(target,
 292                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 293                      info, count / sizeof(struct user_desc));
 294
 295         return 0;
 296 }