Bug 1596795 (CVE-2018-10877)

Summary: CVE-2018-10877 kernel: out-of-bound access in ext4_ext_drop_refs function with a crafted ext4 image
Product: [Other] Security Response Reporter: Laura Pardo <lpardo>
Component: vulnerabilityAssignee: Red Hat Product Security <security-response-team>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: abhgupta, airlied, aquini, bhu, bskeggs, dbaker, dhoward, ewk, fhrbata, hdegoede, hkrzesin, hwkernel-mgr, iboverma, ichavero, itamar, jarodwilson, jforbes, jglisse, jkacur, john.j5live, jokerman, jonathan, josef, jross, jwboyer, kernel-maint, kernel-mgr, labbott, lgoncalv, linville, matt, mchehab, mcressma, mjg59, nmurray, plougher, rvrbovsk, skozina, slawomir, steved, sthangav, trankin, vdronov, williams
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard: impact=low,public=20180416,reported=20180628,source=internet,cvss3=5.2/CVSS:3.0/AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H,cwe=CWE-125,rhel-5/kernel=notaffected,rhel-6/kernel=notaffected,rhel-7/kernel=notaffected,rhel-7/kernel-rt=notaffected,mrg-2/realtime-kernel=notaffected,rhel-alt-7/kernel-alt=affected,rhel-8/kernel=notaffected,fedora-all/kernel=affected,openshift-online-3/kernel=notaffected
Fixed In Version: Doc Type: If docs needed, set a value
Doc Text:
A flaw was found in the Linux kernel ext4 filesystem. An out-of-bound access is possible in the ext4_ext_drop_refs() function when operating on a crafted ext4 filesystem image.
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Clone Of: Environment:
Last Closed: Type: ---
Regression: --- Mount Type: ---
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---
Bug Depends On: 1596797, 1596813, 1602821    
Bug Blocks: 1596798    

Description Laura Pardo 2018-06-29 17:54:43 UTC
A flaw was found in Linux kernel ext4 filesystem. An out-of-bound access is possible in the ext4_ext_drop_refs() function when operating on a crafted ext4 filesystem image.

References:

https://bugzilla.kernel.org/show_bug.cgi?id=199417

http://patchwork.ozlabs.org/patch/929744/

An upstream patch:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bc890a60247171294acc0bd67d211fa4b88d40ba

Comment 1 Laura Pardo 2018-06-29 17:55:56 UTC
Created kernel tracking bugs for this issue:

Affects: fedora-all [bug 1596797]

Comment 4 Justin M. Forbes 2018-07-16 14:34:24 UTC
This is fixed for Fedora with the 4.17.6 stable kernel update

Comment 8 Vladis Dronov 2018-07-18 15:07:38 UTC
Notes:

While the flaw reproducer works when run as a privileged user (the "root"), this requires a mount of a certain filesystem image. An unprivileged attacker cannot do this even from a user+mount namespace:

$ unshare -U -r -m
# mount -t ext4 fs.img mnt/
mount: mnt/: mount failed: Operation not permitted.

The article https://lwn.net/Articles/652468/ discusses unprivileged user mounts and hostile filesystem images:
 
> ... for the most part, the mount() system call is denied to processes running
> within user namespaces, even if they are privileged in their namespaces.

It also states that unprivileged filesystem mounts are not allowed as of now in the Linux kernel and probably won't be allowed in a future. Until that such flaws are considered as not exploitable:

> There were no proposals for solutions to the hostile-filesystem problem.
> But, in the absence of some sort of assurance that they can be made safe,
> unprivileged filesystem mounts are unlikely to gain acceptance; even if the
> feature gets into the kernel, distributions would be likely to disable it.

On the other hand, there is a potential possibility that still an attacker can trick a regular user to mount a malicious filesystem image, like trick him to insert an usb-flash-drive with a forged filesystem to a desktop system which will auto-mount it. In case this results only in a system crash (a DoS due to, for example, a NULL pointer dereference) the flaw impact is low but it still exists. In case of a flaw which results in a privilege escalation the flaw's impact is higher.

So the Red Hat would still consider bugs which require mounting a filesystem image to exploit as security flaws, though with Low severity.

Comment 9 errata-xmlrpc 2018-10-30 09:02:29 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 7

Via RHSA-2018:2948 https://access.redhat.com/errata/RHSA-2018:2948