[PATCH] bug: Exclude non-BUG/WARN exceptions from report_bug()

[Kees Cook <keescook@chromium.org>]

Commit:

  b8347c219649 ("x86/debug: Handle warnings before the notifier chain, to fix KGDB crash")

changed the ordering of fixups, and did not take into account the case of
x86 processing non-WARN() and non-BUG() exceptions. This would lead to
output of a false BUG line with no other information. In the case of
a refcount exception, it would be immediately followed by the refcount
WARN(), producing very strange double-"cut here":

lkdtm: attempting bad refcount_inc() overflow
------------[ cut here ]------------
Kernel BUG at 0000000065f29de5 [verbose debug info unavailable]
------------[ cut here ]------------
refcount_t overflow at lkdtm_REFCOUNT_INC_OVERFLOW+0x6b/0x90 in cat[3065], uid/euid: 0/0
WARNING: CPU: 0 PID: 3065 at kernel/panic.c:657 refcount_error_report+0x9a/0xa4
...

In the prior ordering, exceptions were searched first:

 do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
 ...
                if (fixup_exception(regs, trapnr))
                        return 0;

-               if (fixup_bug(regs, trapnr))
-                       return 0;
-

As a result, fixup_bugs()'s is_valid_bugaddr() didn't take into account
needing to search the exception list first, since that had already
happened.

So, instead of searching the exception list twice (once in
is_valid_bugaddr() and then again in fixup_exception()), just add a
simple sanity check to report_bug() that will immediately bail out
if a BUG() (or WARN()) entry is not found.

Fixes: b8347c219649 ("x86/debug: Handle warnings before the notifier chain, to fix KGDB crash")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 lib/bug.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/bug.c b/lib/bug.c
index c1b0fad31b10..551e4d405307 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -150,6 +150,8 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 		return BUG_TRAP_TYPE_NONE;
 
 	bug = find_bug(bugaddr);
+	if (!bug)
+		return BUG_TRAP_TYPE_NONE;
 
 	file = NULL;
 	line = 0;
-- 
2.7.4


-- 
Kees Cook
Pixel Security