4.15-stable review patch.  If anyone has any objections, please let me know.


From: Kees Cook <keesc...@chromium.org>

commit 1b4cfe3c0a30dde968fb43c577a8d7e262a145ee upstream.

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

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 

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

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.

Link: http://lkml.kernel.org/r/20180301225934.GA34350@beast
Fixes: b8347c219649 ("x86/debug: Handle warnings before the notifier chain, to 
fix KGDB crash")
Signed-off-by: Kees Cook <keesc...@chromium.org>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Richard Weinberger <richard.weinber...@gmail.com>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

 lib/bug.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/lib/bug.c
+++ b/lib/bug.c
@@ -150,6 +150,8 @@ enum bug_trap_type report_bug(unsigned l
                return BUG_TRAP_TYPE_NONE;
        bug = find_bug(bugaddr);
+       if (!bug)
+               return BUG_TRAP_TYPE_NONE;
        file = NULL;
        line = 0;

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