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Date: Thu, 9 Apr 2020 01:01:31 +0300
From: Alexander Popov <>
To: Julia Lawall <>, Gilles Muller
 <>, Nicolas Palix <>,
 Michal Marek <>,,
 "" <>,
 Jann Horn <>, Kees Cook <>,
 Hans Verkuil <>, Mauro Carvalho Chehab
 <>, Linux Media Mailing List
 <>, LKML <>
Subject: Coccinelle rule for CVE-2019-18683


Some time ago I fixed CVE-2019-18683 in the V4L2 subsystem of the Linux kernel.

I created a Coccinelle rule that detects that bug pattern. Let me show it.

Bug pattern

CVE-2019-18683 refers to three similar vulnerabilities caused by the same
incorrect approach to locking that is used in vivid_stop_generating_vid_cap(),
vivid_stop_generating_vid_out(), and sdr_cap_stop_streaming().

For fixes please see the commit 6dcd5d7a7a29c1e4 (media: vivid: Fix wrong
locking that causes race conditions on streaming stop).

These three functions are called during streaming stopping with vivid_dev.mutex
locked. And they all do the same mistake while stopping their kthreads, which
need to lock this mutex as well. See the example from
    /* shutdown control thread */
    vivid_grab_controls(dev, false);
    dev->kthread_vid_cap = NULL;

But when this mutex is unlocked, another vb2_fop_read() can lock it instead of
the kthread and manipulate the buffer queue. That causes use-after-free.

I created a Coccinelle rule that detects mutex_unlock+kthread_stop+mutex_lock
within one function.

Coccinelle rule

virtual report

@race exists@
expression E;
position stop_p;
position unlock_p;
position lock_p;


stop_p << race.stop_p;
unlock_p << race.unlock_p;
lock_p << race.lock_p;
E << race.E;
@@[0], 'mutex_unlock(' + E + ') here')[0], 'kthread_stop here')[0], 'mutex_lock(' + E + ') here\n')

Testing the rule

I reverted the commit 6dcd5d7a7a29c1e4 and called:
COCCI=./scripts/coccinelle/kthread_race.cocci make coccicheck MODE=report

The result:

./drivers/media/platform/vivid/vivid-kthread-out.c:347:1-13: mutex_unlock(& dev
-> mutex) here
./drivers/media/platform/vivid/vivid-kthread-out.c:348:1-13: kthread_stop here
./drivers/media/platform/vivid/vivid-kthread-out.c:350:1-11: mutex_lock(& dev ->
mutex) here

./drivers/media/platform/vivid/vivid-sdr-cap.c:306:1-13: mutex_unlock(& dev ->
mutex) here
./drivers/media/platform/vivid/vivid-sdr-cap.c:307:1-13: kthread_stop here
./drivers/media/platform/vivid/vivid-sdr-cap.c:309:1-11: mutex_lock(& dev ->
mutex) here

./drivers/media/platform/vivid/vivid-kthread-cap.c:1001:1-13: mutex_unlock(& dev
-> mutex) here
./drivers/media/platform/vivid/vivid-kthread-cap.c:1002:1-13: kthread_stop here
./drivers/media/platform/vivid/vivid-kthread-cap.c:1004:1-11: mutex_lock(& dev
-> mutex) here

There are no other bugs detected.

Do you have any idea how to improve it?
Do we need that rule for regression testing in the upstream?

Thanks in advance!

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