Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6

[Peter Zijlstra]

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.