[tip:x86/asm] x86/entry/traps: Clear TIF_BLOCKSTEP on all debug exceptions

[tip-bot for Andy Lutomirski]

Commit-ID:  81edd9f69a6fd214fdbe66b43de6aa1610c84c63
Gitweb:     http://git.kernel.org/tip/81edd9f69a6fd214fdbe66b43de6aa1610c84c63
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 9 Mar 2016 19:00:28 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 10 Mar 2016 09:48:13 +0100

x86/entry/traps: Clear TIF_BLOCKSTEP on all debug exceptions

The SDM says that debug exceptions clear BTF, and we need to keep
TIF_BLOCKSTEP in sync with BTF.  Clear it unconditionally and improve
the comment.

I suspect that the fact that kmemcheck could cause TIF_BLOCKSTEP not
to be cleared was just an oversight.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/fa86e55d196e6dde5b38839595bde2a292c52fdc.1457578375.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/traps.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 410e8e2..1af5621 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -598,6 +598,13 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	dr6 &= ~DR6_RESERVED;
 
 	/*
+	 * 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_tsk_thread_flag(tsk, 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.
@@ -612,11 +619,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	/* DR6 may or may not be cleared by the CPU */
 	set_debugreg(0, 6);
 
-	/*
-	 * The processor cleared BTF, so don't mark that we need it set.
-	 */
-	clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
-
 	/* Store the virtualized DR6 value */
 	tsk->thread.debugreg6 = dr6;