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);
+
+ /*
+ * 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.
