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x86/debug: Fix BTF handling 

The SDM states that #DB clears DEBUGCTLMSR_BTF, this means that when the
bit is set for userspace (TIF_BLOCKSTEP) and a kernel #DB happens first,
the BTF bit meant for userspace execution is lost.

Have the kernel #DB handler restore the BTF bit when it was requested
for userspace.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kyle Huey <me@kylehuey.com> 
Link: https://lore.kernel.org/r/20201027093607.956147736@infradead.org
This commit is contained in:
Peter Zijlstra 2020-10-27 10:15:05 +01:00 committed by Thomas Gleixner
parent ed8780e3f2
commit 2a9baf5ad4
1 changed files with 21 additions and 7 deletions
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28
arch/x86/kernel/traps.c
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@ -799,13 +799,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
*/
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 +866,20 @@ 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 +942,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
irqentry_enter_from_user_mode(regs);
instrumentation_begin();
/*
* 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.