[Precise][CVE-2013-1819] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem end

[Luis Henriques]

Hi Thomas,

Thomas GL <tom.gl at free.fr> writes:

> Hi,
>
> It seems that this patch actually breaks xfs_growfs on kernels prior to
> 3.8.0, and should not have been backported as-is.
> This launchpad bug describes the issue I've seen too:
> https://bugs.launchpad.net/ubuntu/+source/xfsprogs/+bug/1236041
>
> Here is a reference explaining why it was not meant to be backported
> to pre-3.8 kernels:
> http://oss.sgi.com/archives/xfs/2013-02/msg00201.html
> And the corresponding revert in 3.7.9 (after it was added in a
> previous revision):
> https://lkml.org/lkml/2013/2/15/638
>
> Also, there is an alternative patch in this SUSE bug which might be a
> better option for fixing CVE-2013-1819 on kernels prior to 3.8:
> https://bugzilla.novell.com/show_bug.cgi?id=807471#c25
> (not tested here, and must at least be edited since the patch is for
> 3.0, when xfs_buf.c was in a "linux-2.6" subdirectory)

Thank you for bringing this issue to my attention and for providing
all these references.  I will revisit this backport and report back on
bug #1236041.

Cheers,
-- 
Luis


>
> Thanks,
> Thomas.
>
>
> On 2013/09/04, Luis Henriques <luis.henriques at canonical.com> wrote:
>
>> From: Dave Chinner <dchinner at redhat.com>
>> 
>> CVE-2013-1819
>> 
>> BugLink: http://bugs.launchpad.net/bugs/1151527
>> 
>> When _xfs_buf_find is passed an out of range address, it will fail
>> to find a relevant struct xfs_perag and oops with a null
>> dereference. This can happen when trying to walk a filesystem with a
>> metadata inode that has a partially corrupted extent map (i.e. the
>> block number returned is corrupt, but is otherwise intact) and we
>> try to read from the corrupted block address.
>> 
>> In this case, just fail the lookup. If it is readahead being issued,
>> it will simply not be done, but if it is real read that fails we
>> will get an error being reported.  Ideally this case should result
>> in an EFSCORRUPTED error being reported, but we cannot return an
>> error through xfs_buf_read() or xfs_buf_get() so this lookup failure
>> may result in ENOMEM or EIO errors being reported instead.
>> 
>> Signed-off-by: Dave Chinner <dchinner at redhat.com>
>> Reviewed-by: Brian Foster <bfoster at redhat.com>
>> Reviewed-by: Ben Myers <bpm at sgi.com>
>> Signed-off-by: Ben Myers <bpm at sgi.com>
>> (back ported from commit eb178619f930fa2ba2348de332a1ff1c66a31424)
>> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
>> ---
>>  fs/xfs/xfs_buf.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>> 
>> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
>> index 2f5a8f7..c13fff4 100644
>> --- a/fs/xfs/xfs_buf.c
>> +++ b/fs/xfs/xfs_buf.c
>> @@ -434,6 +434,7 @@ _xfs_buf_find(
>>  	struct rb_node		**rbp;
>>  	struct rb_node		*parent;
>>  	xfs_buf_t		*bp;
>> +	xfs_daddr_t		eofs;
>>  
>>  	range_base = (ioff << BBSHIFT);
>>  	range_length = (isize << BBSHIFT);
>> @@ -442,6 +443,23 @@ _xfs_buf_find(
>>  	ASSERT(!(range_length < (1 << btp->bt_sshift)));
>>  	ASSERT(!(range_base & (xfs_off_t)btp->bt_smask));
>>  
>> +	/*
>> +	 * Corrupted block numbers can get through to here,
>> unfortunately, so we
>> +	 * have to check that the buffer falls within the filesystem
>> bounds.
>> +	 */
>> +	eofs = XFS_FSB_TO_BB(btp->bt_mount,
>> btp->bt_mount->m_sb.sb_dblocks);
>> +	if (ioff >= eofs) {
>> +		/*
>> +		 * XXX (dgc): we should really be returning
>> EFSCORRUPTED here,
>> +		 * but none of the higher level infrastructure
>> supports
>> +		 * returning a specific error on buffer lookup
>> failures.
>> +		 */
>> +		xfs_alert(btp->bt_mount,
>> +			  "%s: Block out of range: block 0x%llx,
>> EOFS 0x%llx ",
>> +			  __func__, ioff, eofs);
>> +		return NULL;
>> +	}
>> +
>>  	/* get tree root */
>>  	pag = xfs_perag_get(btp->bt_mount,
>>  				xfs_daddr_to_agno(btp->bt_mount,
>> ioff));