linux-kernel - [PATCH 1/3] x86/debug: Fix BTF handling

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Message-ID: <20201027093607.956147736@infradead.org>
Date:   Tue, 27 Oct 2020 10:15:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, luto@...nel.org, me@...ehuey.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, rocallahan@...il.com,
        alexandre.chartre@...cle.com, paulmck@...nel.org,
        frederic@...nel.org, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, mhiramat@...nel.org,
        pmladek@...e.com, joel@...lfernandes.org, rostedt@...dmis.org,
        boris.ostrovsky@...cle.com, jgross@...e.com, brgerst@...il.com,
        jpoimboe@...hat.com, daniel.thompson@...aro.org,
        julliard@...ehq.org, pgofman@...eweavers.com, peterz@...radead.org
Subject: [PATCH 1/3] x86/debug: Fix BTF handling

The SDM states that #DB clears DEBUGCTLMSR_BTF, this means that when
we set that bit for userspace (TIF_BLOCKSTEP) and get a kernel #DB
first, we'll loose the BTF bit meant for userspace execution.

Have the kernel #DB handler restore the BTF bit when it was requested
for userspace.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/traps.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -799,13 +799,6 @@ static __always_inline unsigned long deb
 	 */
 	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 +866,20 @@ static __always_inline void exc_debug_ke
 	 */
 	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.
@@ -936,6 +943,13 @@ static __always_inline void exc_debug_us
 	instrumentation_begin();
 
 	/*
+	 * 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.
 	 * User wants a sigtrap for that.