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>

1 files changed, 7 insertions, 5 deletions

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 410e8e2700c5..1af56217fb5d 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;