x86: Ignore trap bits on single step exceptions

commit 6c0aca288e726405b01dacb12cac556454d34b2a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index a8f1b803d2fd916c7aacf4952ff33b620f790852..f3654702099a9c1d1817bdf4b22ce501c7b58d5d 100644 (file)
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -421,6 +421,10 @@ 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;