x86: Ignore trap bits on single step exceptions

When a single step exception fires, the trap bits, used to
signal hardware breakpoints, are in a random state.

These trap bits might be set if another exception will follow,
like a breakpoint in the next instruction, or a watchpoint in the
previous one. Or there can be any junk there.

So if we handle these trap bits during the single step exception,
we are going to handle an exception twice, or we are going to
handle junk.

Just ignore them in this case.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=21332

Reported-by: Michael Stefaniuc <mstefani@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Maciej Rutecki <maciej.rutecki@gmail.com>
Cc: Alexandre Julliard <julliard@winehq.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: All since 2.6.33.x <stable@kernel.org>

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index ff15c9dcc25de8be8144fd4d15f1dfd3069314e4..42c59425450727284615b2a920611e21047ce715 100644 (file)
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -433,6 +433,10 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
        dr6_p = (unsigned long *)ERR_PTR(args->err);
        dr6 = *dr6_p;
 
+       /* If it's a single step, TRAP bits are random */
+       if (dr6 & DR_STEP)
+               return NOTIFY_DONE;
+
        /* Do an early return if no trap bits are set in DR6 */
        if ((dr6 & DR_TRAP_BITS) == 0)
                return NOTIFY_DONE;