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Subject[tip:x86/asm] x86/entry/traps: Clear TIF_BLOCKSTEP on all debug exceptions
Commit-ID:  81edd9f69a6fd214fdbe66b43de6aa1610c84c63
Gitweb: http://git.kernel.org/tip/81edd9f69a6fd214fdbe66b43de6aa1610c84c63
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Wed, 9 Mar 2016 19:00:28 -0800
Committer: Ingo Molnar <mingo@kernel.org>
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@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/fa86e55d196e6dde5b38839595bde2a292c52fdc.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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;

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