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Date: Thu, 4 Jun 2020 16:09:59 +0200
From: Matthias Gerstner <>
Subject: xawtv: CVE-2020-13696: v4l-conf setuid-root program allows file
 existence tests and open(..., O_RDRW) on arbitrary files


xawtv [1] contains a setuid-root program called `v4l-conf` that is
supposed to allow regular users to configure v4l devices. xawtv is
pretty old code but it is stilled shipped on some distributions like
Debian and openSUSE.

Vulnerability Description

While checking the source code of `v4l-conf` I noticed that it allows
regular users to perform arbitrary file existence tests and also to
perform `open(..., O_RDWR)` and `fstat()` system calls on arbitrary
files. The issue is found in the `dev_open()` function which only
contains a naive security check:

    if (strncmp(device, "/dev/", 5)) {
	fprintf(stderr, "error: %s is not a /dev file\n", device);

This check is not safe against relative path components or symlinks in
/dev/shm. Example:

# in this case the file does not exist
user $ v4l-conf -c /dev/../root/.bashrc
VT_GETSTATE is not supported: Inappropriate ioctl for device
mode: 0x0, depth=0, bpp=0, bpl=0, base=unknown
can't open /dev/../root/.bashrc: No such file or directory

# in this case the file exists
user $ v4l-conf -c /dev/../root/.bash_history
VT_GETSTATE is not supported: Inappropriate ioctl for device
mode: 0x0, depth=0, bpp=0, bpl=0, base=unknown
/dev/../root/.bash_history: wrong device

Some devices in /dev might also trigger code paths upon open() in the
kernel that are usually not reachable to regular users.


Upstream added two fixes in their Git repository [2]:

- commit 31f31f9cbaee7be806cba38e0ff5431bd44b20a3
- commit 36dc44e68e5886339b4a0fbe3f404fb1a4fd2292

The fix is still incomplete though. It avoids to perform an `open()` on
unintended files. But it still allows the file existence test to be
performed. Attached is a small patch that fixes also that.


2020-05-14: I privately reported the finding to
  , one of the xawtv maintainers
	    denoted in [3].
2020-05-14 - 2020-05-29: various discussions with the maintainer about
            the fix. He published the partial fixes in the Git
	    repository right away.
2020-05-29  I reported to the maintainer that the fix is still
            incomplete but did not hear back since then.
2020-05-29  I requested and obtained a CVE from Mitre for this issue.




Matthias Gerstner <>
Dipl.-Wirtsch.-Inf. (FH), Security Engineer
Phone: +49 911 740 53 290
GPG Key ID: 0x14C405C971923553

SUSE Software Solutions Germany GmbH
HRB 36809, AG Nürnberg
Geschäftsführer: Felix Imendörffer

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