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>

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 1af56217fb5dccf09a96a46fc2c6cc3190c6e0b2..7394be8b1a669d71cd807ee91251fd4a628afe81 100644 (file)
--- 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;