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Date: Tue, 29 Mar 2016 16:56:11 -0600
From: Andreas Dilger <adilger@...ger.ca>
To: Yves-Alexis Perez <corsac@...ian.org>
Cc: 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 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?

Cheers, Andreas

>> The crafted image may be burn onto SD card or USB key to crash a large
>> panel of linux box.
>> 
>> 
>> [ 929.200197] EXT4-fs error (device loop0): ext4_iget:4058: inode #2: comm
>> mount: bad extended attribute block 8390656
>> [ 929.200226] Kernel panic - not syncing: EXT4-fs (device loop0): panic
>> forced after error
>> [ 929.200226]
>> [ 929.200230] CPU: 1 PID: 980 Comm: mount Tainted: G O
>> 3.18.17-8.pvops.qubes.x86_64 #1
>> [ 929.200233] 0000000000000000 000000007533690c ffff88000ea07aa8
>> ffffffff81722191
>> [ 929.200237] 0000000000000000 ffffffff81a84108 ffff88000ea07b28
>> ffffffff8171a462
>> [ 929.200240] ffff880000000010 ffff88000ea07b38 ffff88000ea07ad8
>> 000000007533690c
>> [ 929.200244] Call Trace:
>> [ 929.200249] [<ffffffff81722191>] dump_stack+0x46/0x58
>> [ 929.200253] [<ffffffff8171a462>] panic+0xd0/0x204
>> [ 929.200257] [<ffffffff812ae4d6>] ext4_handle_error.part.188+0x96/0xa0
>> [ 929.200260] [<ffffffff812ae838>] __ext4_error_inode+0xa8/0x180
>> [ 929.200264] [<ffffffff81292869>] ext4_iget+0x929/0xae0
>> [ 929.200267] [<ffffffff812b31fb>] ext4_fill_super+0x18db/0x2b60
>> [ 929.200270] [<ffffffff8120af20>] mount_bdev+0x1b0/0x1f0
>> [ 929.200273] [<ffffffff812b1920>] ? ext4_calculate_overhead+0x3d0/0x3d0
>> [ 929.200276] [<ffffffff812a3425>] ext4_mount+0x15/0x20
>> [ 929.200278] [<ffffffff8120b879>] mount_fs+0x39/0x1b0
>> [ 929.200282] [<ffffffff811afd95>] ? __alloc_percpu+0x15/0x20
>> [ 929.200285] [<ffffffff8122754b>] vfs_kern_mount+0x6b/0x110
>> [ 929.200287] [<ffffffff8122a38c>] do_mount+0x22c/0xb60
>> [ 929.200290] [<ffffffff811aab96>] ? memdup_user+0x46/0x80
>> [ 929.200292] [<ffffffff8122b002>] SyS_mount+0xa2/0x110
>> [ 929.200295] [<ffffffff8172a609>] system_call_fastpath+0x12/0x17
>> [ 929.200301] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation
>> range: 0xffffffff80000000-0xffffffff9fffffff)c
>> 
>> I cannot attach the PoC (2x2MB too large) nor sending it in plain text
>> (they are filesystems), so I've uploaded it on this website of free file
>> sharing ... (sorry for the inconvenient):
>> poc.ext2 https://1fichier.com/?zbk2gohk8s
>> poc.ext3 https://1fichier.com/?9r0c8agjfa
>> 
>> Can you assign a CVE for this?
>> Thank for reading and your time.
>> 
>> Hugues ANGUELKOV.
>> 
>> 
> --
> Yves-Alexis
> 


Cheers, Andreas






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