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Date: Thu, 25 Aug 2016 13:49:10 -0400 (EDT)
From: cve-assign@...re.org
To: caiqian@...hat.com
Cc: cve-assign@...re.org, oss-security@...ts.openwall.com
Subject: Re: cve request: overlayfs: Fix dentry reference leak

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> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ab79efab0a0ba01a74df782eb7fa44b044dae8b5
> 
> In ovl_copy_up_locked(), newdentry is leaked if the function exits through
> out_cleanup as this just to out after calling ovl_cleanup() - which doesn't
> actually release the ref on newdentry.
> 
> The out_cleanup segment should instead exit through out2 as certainly
> newdentry leaks - and possibly upper does also, though this isn't caught
> given the catch of newdentry.
> 
> Without this fix, something like the following is seen:
> 
>         BUG: Dentry ffff880023e9eb20{i=f861,n=#ffff880023e82d90} still in use (1) [unmount of tmpfs tmpfs]
>         BUG: Dentry ffff880023ece640{i=0,n=bigfile}  still in use (1) [unmount of tmpfs tmpfs]
> 
> when unmounting the upper layer after an error occurred in copyup.
> 
> An error can be induced by creating a big file in a lower layer with
> something like:
> 
>         dd if=/dev/zero of=/lower/a/bigfile bs=65536 count=1 seek=$((0xf000))
> 
> to create a large file (4.1G). Overlay an upper layer that is too small
> (on tmpfs might do) and then induce a copy up by opening it writably.
> 
> === POC Exploit ===
> This can be reproduced in a DevOps environment when the docker runtime storage is on overlayfs over
> xfs as a local DoS. An attacker access to a developer account could run a crafted image from elsewhere
> like docker by creating a big file in the container filesystem and try to read it running by any
> user like below by forcing xfs_file_open() returns -EFBIG,
> 
> $ cat Dockerfile
> FROM fedora
> USER nobody
> RUN dd if=/dev/zero of=/home/nobody/bigfile bs=1024k seek=2046 count=1
> ADD open /home/nobody
> CMD ["/home/nobody/open", "/home/nobody/bigfile"]
> 
> and possibly trigger kernel dentry leaks inside the container that will eventually running out of
> kernel resources for other developers. Hence, a local DoS.

Use CVE-2015-8953.

- -- 
CVE Assignment Team
M/S M300, 202 Burlington Road, Bedford, MA 01730 USA
[ A PGP key is available for encrypted communications at
  http://cve.mitre.org/cve/request_id.html ]
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