linux-kernel - [PATCH] bug: Exclude non-BUG/WARN exceptions from report

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Message-ID: <20180225165251.GA11764@beast>
Date:   Sun, 25 Feb 2018 08:52:51 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Richard Weinberger <richard.weinberger@...il.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] bug: Exclude non-BUG/WARN exceptions from report_bug()

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@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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