Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 26 Oct 2020 16:30:32 -0700 | Subject | Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6 |
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On Mon, Oct 26, 2020 at 9:55 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Oct 26, 2020 at 05:31:00PM +0100, Peter Zijlstra wrote: > > In that respect, I think the current virtual_dr6 = 0 is placed wrong, it > > should only be in exc_debug_user(). The only 'problem' then is that we > > seem to be able to loose BTF, but perhaps that is already an extant bug. > > > > Consider: > > > > - perf: setup in-kernel #DB > > - tracer: ptrace(PTRACE_SINGLEBLOCK) > > - tracee: #DB on perf breakpoint, looses BTF > > - tracee .. never triggers actual blockstep > > > > Hmm ? Should we re-set BTF when TIF_BLOCKSTEP && !user_mode(regs) ? > > Something like so then. > > Or sould we also have the userspace #DB re-set BTF when it was !DR_STEP? > I need to go untangle that ptrace stuff :/ > > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c > index 3c70fb34028b..31de8b0980ca 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 +936,26 @@ static __always_inline void exc_debug_user(struct pt_regs *regs, > irqentry_enter_from_user_mode(regs); > instrumentation_begin(); > > + /* > + * Clear the virtual DR6 value, ptrace routines will set bits here for > + * things we want signals for. > + */ > + current->thread.virtual_dr6 = 0; > + > + /* > + * If PTRACE requested SINGLE(BLOCK)STEP, make sure to reflect that in > + * the ptrace visible DR6 copy. > + */ > + if (test_thread_flag(TIF_BLOCKSTEP) || test_thread_flag(TIF_SINGLESTEP)) > + current->thread.virtual_dr6 |= (dr6 & DR_STEP);
I'm guessing that this would fail a much simpler test, though: have a program use PUSHF to set TF and then read out DR6 from the SIGTRAP. I can whip up such a test if you like.
Is there any compelling reason not to just drop the condition and do:
current->thread.virtual_dr6 |= (dr6 & DR_STEP);
unconditionally? This DR6 cause, along with ICEBP, have the regrettable distinctions of being the only causes that a user program can trigger all on its own without informing the kernel first. This means that we can't fully separate the concept of "user mode is single-stepping itself" from "ptrace or something else is causing the kernel to single step a program."
I bet that, without making this tweak, the virtual_dr6 change will regress some horrific Wine use case.
--Andy
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