Messages in this thread Patch in this message | | | Date | Sun, 01 Nov 2020 13:56:04 -0000 | From | Thomas Gleixner <> | Subject | [GIT pull] x86/urgent for v5.10-rc2 |
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Linus,
please pull the latest x86/urgent branch from:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-urgent-2020-11-01
up to: cb05143bdf42: x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
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.
Thanks,
tglx
------------------> Peter Zijlstra (3): x86/debug: Fix BTF handling x86/debug: Only clear/set ->virtual_dr6 for userspace #DB x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
arch/x86/kernel/traps.c | 43 ++++++++++++++++++++++++++++++------------- 1 file changed, 30 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 3c70fb34028b..e19df6cde35d 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -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.
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