Date: Thu, 31 Mar 2016 09:41:28 -0500 From: Eric Sandeen <sandeen@...hat.com> To: "Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger@...ger.ca> Cc: Yves-Alexis Perez <corsac@...ian.org>, oss-security@...ts.openwall.com, Theodore Tso <tytso@...gle.com>, linux-ext4@...r.kernel.org Subject: Re: CVE Request - Linux kernel (multiple versions) ext2/ext3 filesystem DoS On 3/30/16 3:43 PM, Theodore Ts'o wrote: > On Tue, Mar 29, 2016 at 04:56:11PM -0600, Andreas Dilger wrote: >> On Mar 29, 2016, at 3:14 PM, Yves-Alexis Perez <corsac@...ian.org> wrote: >>> >>> [dropping MITRE from CC since it's not about the CVE] >>> [adding ext and Theodore to CC] >>> >>> On mar., 2016-03-29 at 19:24 +0200, Hugues ANGUELKOV wrote: >>>> Hello, >>>> >>>> The linux kernel is prone to a Denial of service when mounting specially >>>> crafted ext2/ext3 (possibly ext4) filesystems. This occurs in the function >>>> ext4_handle_error who call the panic function on precise circumstance. >>> >>> Did you contact the upstream maintainers about this? I'm adding them just in >>> case they're not already aware of that… >>> >>>> This was tested on severals linux kernel version: 3.10, 3.18, 3.19, on >>>> real hardware and Xen DomU PV & HVM (the crash report attached is from a >>>> Fedora 3.18 PV DomU), from different distribution release: Ubuntu, CentOS, >>>> Fedora, Linux Mint, QubesOS. >>>> This a low security impact bug, because generally only root can mount >>>> image, however on Desktop (or possibly server?) system configured with >>>> automount the bug is easily triggable (think of android smartphone? Haven't >>>> test yet). >> >> It seems that the important point here is that the filesystem has >> "s_errors=EXT4_ERRORS_PANIC" set in the superblock? I don't think >> the actual corruption that triggered the ext4_error() call is important, >> since there are any number of other failure cases that could generate >> a similar error. >> >> It seems practical to change s_errors at mount time from EXT4_ERRORS_PANIC >> to EXT4_ERRORS_RO for filesystems mounted by regular users. The question >> is whether there is a way for the ext4 code to know this at mount time? > > You can mount the file system with "mount -o errors=continue" and this > will override the default behavior specified in the super block. > > I would argue that a Desktop or server system that had automount > should either (a) mount with -o errors=continue, or (b) force an fsck > on the file system before mounting it. > > So I think this is a particularly meaningless CVE, which is why I have > zero respect for people who try to make any kind of conclusion based > on CVE counts. I certainly don't plan to do anything about this. > > You might as well complain that since the system ships with a reboot > command that can be executed by a clueless root user, that this is a > potential DOS attack scenario deserving of a CVE.... First of all, yes, I have always been extremely skeptical of these "crafted image" CVEs. However, I'm not sure the "store errors=panic in the superblock" was particularly well thought out either; it certainly does make for a tidy little timebomb. While I really hate to give issues such as this a whole lot more credibility, I wonder about a higher level control, such as a sysctl, which could [dis]allow errors=panic at a system-wide level. It could default to disallowing, and it's trivial to set it in sysctl.conf if you really want it enabled by default. In the end, errors=panic is really a debug option; a small hoop-jump to use it doesn't sound too bad to me. -Eric
Powered by blists - more mailing lists
Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.