From: Kees Cook keescook@chromium.org Subject: lib/bug.c: exclude non-BUG/WARN exceptions from report_bug()

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.

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 keescook@chromium.org Cc: Ingo Molnar mingo@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: Borislav Petkov bp@alien8.de Cc: Richard Weinberger richard.weinberger@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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

diff -puN lib/bug.c~bug-exclude-non-bug-warn-exceptions-from-report_bug lib/bug.c --- a/lib/bug.c~bug-exclude-non-bug-warn-exceptions-from-report_bug +++ a/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; _