[tip:x86/asm] x86/entry/traps: Clear DR6 early in do_debug() and improve the comment

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  8bb5643686d2898bec181c407651cf84e77d413f
Gitweb:     http://git.kernel.org/tip/8bb5643686d2898bec181c407651cf84e77d413f
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Wed, 9 Mar 2016 19:00:29 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 10 Mar 2016 09:48:13 +0100

x86/entry/traps: Clear DR6 early in do_debug() and improve the comment

Leaving any bits set in DR6 on return from a debug exception is
asking for trouble.  Prevent it by writing zero right away and
clarify the comment.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3857676e1be8fb27db4b89bbb1e2052b7f435ff4.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/traps.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 1af5621..7394be8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -593,6 +593,18 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	ist_enter(regs);
 
 	get_debugreg(dr6, 6);
+	/*
+	 * The Intel SDM says:
+	 *
+	 *   Certain debug exceptions may clear bits 0-3. The remaining
+	 *   contents of the DR6 register are never cleared by the
+	 *   processor. To avoid confusion in identifying debug
+	 *   exceptions, debug handlers should clear the register before
+	 *   returning to the interrupted task.
+	 *
+	 * Keep it simple: clear DR6 immediately.
+	 */
+	set_debugreg(0, 6);
 
 	/* Filter out all the reserved bits which are preset to 1 */
 	dr6 &= ~DR6_RESERVED;
@@ -616,9 +628,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
 		goto exit;
 
-	/* DR6 may or may not be cleared by the CPU */
-	set_debugreg(0, 6);
-
 	/* Store the virtualized DR6 value */
 	tsk->thread.debugreg6 = dr6;