[patch 3/8] lib/bug.c: exclude non-BUG/WARN exceptions from report_bug()

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From: Kees Cook <keescook@xxxxxxxxxxxx>
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@xxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Richard Weinberger <richard.weinberger@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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;
_
--
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