linux-kernel - [PATCH 2/3] x86/debug: Only clear/set ->virtual

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Date:   Tue, 27 Oct 2020 10:15:06 +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 2/3] x86/debug: Only clear/set ->virtual_dr6 for userspace #DB

The ->virtual_dr6 is the value used by ptrace_{get,set}_debugreg(6). A
kernel #DB clearing it could mean spurious malfunction of ptrace()
expectations.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/traps.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -793,12 +793,6 @@ static __always_inline unsigned long deb
 	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;
-
 	return dr6;
 }
 
@@ -943,6 +937,12 @@ static __always_inline void exc_debug_us
 	instrumentation_begin();
 
 	/*
+	 * Clear the virtual DR6 value, ptrace() routines will set bits here
+	 * for things it wants 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.