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Three fixes all related to #DB:

Browse Source 
- Handle the BTF bit correctly so it doesn't get lost due to a kernel #DB
 
  - Only clear and set the virtual DR6 value used by ptrace on user space
    triggered #DB. A kernel #DB must leave it alone to ensure data
    consistency for ptrace.
 
  - Make the bitmasking of the virtual DR6 storage correct so it does not
    lose DR_STEP.
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Merge tag 'x86-urgent-2020-11-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Thomas Gleixner:
 "Three fixes all related to #DB:

   - Handle the BTF bit correctly so it doesn't get lost due to a kernel
     #DB

   - Only clear and set the virtual DR6 value used by ptrace on user
     space triggered #DB. A kernel #DB must leave it alone to ensure
     data consistency for ptrace.

   - Make the bitmasking of the virtual DR6 storage correct so it does
     not lose DR_STEP"

* tag 'x86-urgent-2020-11-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
  x86/debug: Only clear/set ->virtual_dr6 for userspace #DB
  x86/debug: Fix BTF handling
This commit is contained in:
Linus Torvalds 2020-11-01 11:21:26 -08:00
commit 7b56fbd83e
1 changed files with 30 additions and 13 deletions
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43
arch/x86/kernel/traps.c
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@ -793,19 +793,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
set_debugreg(DR6_RESERVED, 6);
dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
/*
* Clear the virtual DR6 value, ptrace routines will set bits here for
* things we want signals for.
*/
current->thread.virtual_dr6 = 0;
/*
* The SDM says "The processor clears the BTF flag when it
* generates a debug exception." Clear TIF_BLOCKSTEP to keep
* TIF_BLOCKSTEP in sync with the hardware BTF flag.
*/
clear_thread_flag(TIF_BLOCKSTEP);
return dr6;
}
@ -873,6 +860,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
*/
WARN_ON_ONCE(user_mode(regs));
if (test_thread_flag(TIF_BLOCKSTEP)) {
/*
* The SDM says "The processor clears the BTF flag when it
* generates a debug exception." but PTRACE_BLOCKSTEP requested
* it for userspace, but we just took a kernel #DB, so re-set
* BTF.
*/
unsigned long debugctl;
rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
debugctl |= DEBUGCTLMSR_BTF;
wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
}
/*
* Catch SYSENTER with TF set and clear DR_STEP. If this hit a
* watchpoint at the same time then that will still be handled.
@ -935,6 +936,22 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
irqentry_enter_from_user_mode(regs);
instrumentation_begin();
/*
* Start the virtual/ptrace DR6 value with just the DR_STEP mask
* of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
*
* Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
* even if it is not the result of PTRACE_SINGLESTEP.
*/
current->thread.virtual_dr6 = (dr6 & DR_STEP);
/*
* The SDM says "The processor clears the BTF flag when it
* generates a debug exception." Clear TIF_BLOCKSTEP to keep
* TIF_BLOCKSTEP in sync with the hardware BTF flag.
*/
clear_thread_flag(TIF_BLOCKSTEP);
/*
* If dr6 has no reason to give us about the origin of this trap,
* then it's very likely the result of an icebp/int01 trap.