arch/x86/kernel/fred.c

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #include <linux/kernel.h>
   3
   4 #include <asm/desc.h>
   5 #include <asm/fred.h>
   6 #include <asm/tlbflush.h>
   7 #include <asm/traps.h>
   8
   9 /* #DB in the kernel would imply the use of a kernel debugger. */
  10 #define FRED_DB_STACK_LEVEL             1UL
  11 #define FRED_NMI_STACK_LEVEL            2UL
  12 #define FRED_MC_STACK_LEVEL             2UL
  13 /*
  14  * #DF is the highest level because a #DF means "something went wrong
  15  * *while delivering an exception*." The number of cases for which that
  16  * can happen with FRED is drastically reduced and basically amounts to
  17  * "the stack you pointed me to is broken." Thus, always change stacks
  18  * on #DF, which means it should be at the highest level.
  19  */
  20 #define FRED_DF_STACK_LEVEL             3UL
  21
  22 #define FRED_STKLVL(vector, lvl)        ((lvl) << (2 * (vector)))
  23
  24 DEFINE_PER_CPU(unsigned long, fred_rsp0);
  25 EXPORT_PER_CPU_SYMBOL(fred_rsp0);
  26
  27 void cpu_init_fred_exceptions(void)
  28 {
  29         /* When FRED is enabled by default, remove this log message */
  30         pr_info("Initialize FRED on CPU%d\n", smp_processor_id());
  31
  32         /*
  33          * If a kernel event is delivered before a CPU goes to user level for
  34          * the first time, its SS is NULL thus NULL is pushed into the SS field
  35          * of the FRED stack frame.  But before ERETS is executed, the CPU may
  36          * context switch to another task and go to user level.  Then when the
  37          * CPU comes back to kernel mode, SS is changed to __KERNEL_DS.  Later
  38          * when ERETS is executed to return from the kernel event handler, a #GP
  39          * fault is generated because SS doesn't match the SS saved in the FRED
  40          * stack frame.
  41          *
  42          * Initialize SS to __KERNEL_DS when enabling FRED to avoid such #GPs.
  43          */
  44         loadsegment(ss, __KERNEL_DS);
  45
  46         wrmsrl(MSR_IA32_FRED_CONFIG,
  47                /* Reserve for CALL emulation */
  48                FRED_CONFIG_REDZONE |
  49                FRED_CONFIG_INT_STKLVL(0) |
  50                FRED_CONFIG_ENTRYPOINT(asm_fred_entrypoint_user));
  51
  52         wrmsrl(MSR_IA32_FRED_STKLVLS, 0);
  53         wrmsrl(MSR_IA32_FRED_RSP0, 0);
  54         wrmsrl(MSR_IA32_FRED_RSP1, 0);
  55         wrmsrl(MSR_IA32_FRED_RSP2, 0);
  56         wrmsrl(MSR_IA32_FRED_RSP3, 0);
  57
  58         /* Enable FRED */
  59         cr4_set_bits(X86_CR4_FRED);
  60         /* Any further IDT use is a bug */
  61         idt_invalidate();
  62
  63         /* Use int $0x80 for 32-bit system calls in FRED mode */
  64         setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
  65         setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
  66 }
  67
  68 /* Must be called after setup_cpu_entry_areas() */
  69 void cpu_init_fred_rsps(void)
  70 {
  71         /*
  72          * The purpose of separate stacks for NMI, #DB and #MC *in the kernel*
  73          * (remember that user space faults are always taken on stack level 0)
  74          * is to avoid overflowing the kernel stack.
  75          */
  76         wrmsrl(MSR_IA32_FRED_STKLVLS,
  77                FRED_STKLVL(X86_TRAP_DB,  FRED_DB_STACK_LEVEL) |
  78                FRED_STKLVL(X86_TRAP_NMI, FRED_NMI_STACK_LEVEL) |
  79                FRED_STKLVL(X86_TRAP_MC,  FRED_MC_STACK_LEVEL) |
  80                FRED_STKLVL(X86_TRAP_DF,  FRED_DF_STACK_LEVEL));
  81
  82         /* The FRED equivalents to IST stacks... */
  83         wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB));
  84         wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI));
  85         wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF));
  86 }