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Commit7b56fbd

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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
2 parents4312e0e +cb05143 commit7b56fbd

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‎arch/x86/kernel/traps.c‎

Lines changed: 30 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -793,19 +793,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
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set_debugreg(DR6_RESERVED,6);
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dr6 ^=DR6_RESERVED;/* Flip to positive polarity */
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/*
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* Clear the virtual DR6 value, ptrace routines will set bits here for
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* things we want signals for.
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*/
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current->thread.virtual_dr6=0;
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/*
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* The SDM says "The processor clears the BTF flag when it
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* generates a debug exception." Clear TIF_BLOCKSTEP to keep
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* TIF_BLOCKSTEP in sync with the hardware BTF flag.
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*/
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clear_thread_flag(TIF_BLOCKSTEP);
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returndr6;
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}
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@@ -873,6 +860,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
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*/
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WARN_ON_ONCE(user_mode(regs));
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if (test_thread_flag(TIF_BLOCKSTEP)) {
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/*
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* The SDM says "The processor clears the BTF flag when it
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* generates a debug exception." but PTRACE_BLOCKSTEP requested
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* it for userspace, but we just took a kernel #DB, so re-set
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* BTF.
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*/
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unsigned longdebugctl;
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rdmsrl(MSR_IA32_DEBUGCTLMSR,debugctl);
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debugctl |=DEBUGCTLMSR_BTF;
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wrmsrl(MSR_IA32_DEBUGCTLMSR,debugctl);
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}
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/*
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* Catch SYSENTER with TF set and clear DR_STEP. If this hit a
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* 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,
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irqentry_enter_from_user_mode(regs);
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instrumentation_begin();
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/*
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* Start the virtual/ptrace DR6 value with just the DR_STEP mask
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* of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
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*
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* Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
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* even if it is not the result of PTRACE_SINGLESTEP.
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*/
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current->thread.virtual_dr6= (dr6&DR_STEP);
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/*
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* The SDM says "The processor clears the BTF flag when it
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* generates a debug exception." Clear TIF_BLOCKSTEP to keep
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* TIF_BLOCKSTEP in sync with the hardware BTF flag.
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*/
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clear_thread_flag(TIF_BLOCKSTEP);
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/*
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* If dr6 has no reason to give us about the origin of this trap,
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* then it's very likely the result of an icebp/int01 trap.

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