The oddity of CVE-2014-9940 and the problem of recognizing kernel security patches

June 21, 2017

Let's start with my tweet:

Do I want to know why a reported and disclosed in 2017 Linux kernel vulnerability has a 2014 CVE number? Probably not.

Today, Ubuntu came out with USN-335-1, a security advisory about their Ubuntu 14.04 LTS kernel. Among the collection of CVEs fixed was one that caught my eye, CVE-2014-9940. This was simply because of the '2014' bit, which is normally the year of the CVE. At first I thought this might be Ubuntu's usual thing where they sometimes repeat old, long-patched issues in their update announcements, but no; as far as I can tell this is a new issue. Ubuntu's collection of links led to the May Android security bulletin, which says that CVE-2014-9940 was only reported on February 15th, 2017.

(I think that the Android security bulletin is the first report.)

So where does the 2014 come from? That's where I wound up looking more closely at the kernel commit that fixes it:

Author: Seung-Woo Kim
Date: Thu Dec 4 19:17:17 2014 +0900

regulator: core: Fix regualtor_ena_gpio_free not to access pin after freeing

After freeing pin from regulator_ena_gpio_free, loop can access the pin. So this patch fixes not to access pin after freeing.

This fix was authentically made between 3.18-rc1 and 3.19-rc1, and so it appears that the CVE number was assigned based on when the fix was made, not when the issue was reported.

The next question is why it took until 2017 for vendors using old kernels to patch them against this issue. Although I don't know for sure, I have a theory, namely that this simply wasn't recognized as a security vulnerability until early this year. Many fixes go into every kernel version, far too many to backport them all, so Linux distributions have to pick and choose. Naturally distributions grab security fixes, but that requires everyone involved to actually recognize that what they've written is a security fix. I rather suspect that back in 2014, no one realized that this use-after-free issue was an (exploitable) vulnerability.

It's interesting that this seems to have been reported around the time of CVE-2017-6074, where I started to hear that use-after-free issues in the kernel were increasingly exploitable. I wonder if people went trawling through kernel changelogs to find 'fixed use-after-free issue' changes like this one, then did some digging to see if the issues could be weaponized into vulnerabilities and if any currently used older kernels (such as Android kernels and old Ubuntu LTSes) had missed picking up the patches.

(If people are doing this sort of trawling, I suspect that we can expect a whole series of future CVEs on similar issues.)

If I'm right here, the story of CVE-2014-9940 makes for an excellent example of how not all security fixes are realized to be so at the time (which makes it hard to note them as security fixes in kernel changelogs). As this CVE demonstrates, sometimes it may take more than two years for someone to realize that a particular bugfix closes a security vulnerability. Then everyone with old kernels gets to scramble around to fix them.

(By the way, the answer to this is not 'everyone should run the latest kernel'. Nor is it 'the kernel should stop changing and everyone should focus on removing bugs from it'. The real answer is that there is no solution because this is a hard problem.)

Written on 21 June 2017.
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Last modified: Wed Jun 21 01:30:09 2017
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