Bug 515741 - 2.6.30 kernel stopped supporting xattrs on hugetlbfs
Summary: 2.6.30 kernel stopped supporting xattrs on hugetlbfs
Alias: None
Product: Fedora
Classification: Fedora
Component: kernel
Version: 13
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Virtualization Maintenance
QA Contact: Fedora Extras Quality Assurance
Depends On:
Blocks: F12VirtTarget 513462
TreeView+ depends on / blocked
Reported: 2009-08-05 14:58 UTC by Daniel Berrangé
Modified: 2011-06-27 14:19 UTC (History)
9 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2011-06-27 14:19:47 UTC
Type: ---

Attachments (Terms of Use)
Patch to resolve case relative to 2.6.31-rc5 (2.78 KB, patch)
2009-08-14 05:11 UTC, john cooper
no flags Details | Diff

Description Daniel Berrangé 2009-08-05 14:58:15 UTC
Description of problem:
libvirt needs to use hugetlbfs as the backing store for KVM guest RAM. With sVirt, every KVM guest runs under a dedicated security context. To prevent guests accessing each other's RAM, we need to label files on hugetlbfs matching each guest.

This was possible on 2.6.29 kernels

# mount -t hugetlbfs none /mnt/
# cd /mnt
# touch foo bar
# chcon system_u:object_r:svirt_image_t:s0:c500 bar
# ls -lZ
-rw-r--r--. root root system_u:object_r:svirt_image_t:s0:c500 bar
-rw-r--r--. root root system_u:object_r:hugetlbfs_t:s0 foo
# uname -r

But no longer works on 2.6.30

# mount -t hugetlbfs none /mnt/
# cd /mnt
# touch foo bar
# chcon system_u:object_r:svirt_image_t:s0:c500 bar
chcon: failed to change context of `bar' to `system_u:object_r:svirt_image_t:s0:c500': Operation not supported
# ls -lZ
-rw-r--r--. root root system_u:object_r:hugetlbfs_t:s0 bar
-rw-r--r--. root root system_u:object_r:hugetlbfs_t:s0 foo
# uname -r

There is an explanation for this here:


[quote Sephen Smalley]

> There is a vfs fallback for setxattr of the security.* namespace to the
> security module, which would work for hugetlbfs if not for the fact that
> policy defines it as a genfscon-labeled filesystem.  We only started
> prohibiting setxattr on genfscon-labeled filesystems in 2.6.30; prior to
> that we only did that for mountpoint-labeled filesystems.  I can
> actually chcon a file in a hugetlbfs mount on 2.6.29.

> To convert hugetlbfs to fully support labeling we'd need
> hugetlbfs_mknod() to call security_inode_init_security() to set up new
> inode security labels, just like shmem_mknod() does for tmpfs.  And then
> we'd need to switch over the policy from genfscon to fs_use_trans.

Version-Release number of selected component (if applicable):

How reproducible:

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Expected results:

Additional info:

Comment 1 john cooper 2009-08-14 05:05:12 UTC
The attached patch restores the ability to {set,get}xattr(2)
on hugetlbfs inodes in 2.6.31-rc5 which should resolve this
case.  A tad more infrastructure needed to be pulled in to
support this functionality -- patch is attached but has yet
to be laundered upstream.

Also required is a modification of:


to change hugetlbfs from genfscon to fs_use_xattr (Daniel
Walsh has captured the details).  Without this modification
selinux in enforcing mode will still disallow set/get of
inode xattrs.

Comment 2 john cooper 2009-08-14 05:11:24 UTC
Created attachment 357394 [details]
Patch to resolve case relative to 2.6.31-rc5

Comment 3 Mark McLoughlin 2009-10-06 15:33:48 UTC
John, could you post the latest status on this here? thanks

Comment 4 john cooper 2009-10-08 06:08:01 UTC
Since creating the fix captured in the above patch
I discovered there was another effort in flight
which addressed hubetlbfs xattr support but was more
comprehensive in scope.  I was unsure whether this
fix hit F12 beta as of kernel-2.6.31-
however testing showed the problem still exists here.
Unfortunately no upstream fix is available to backport
as the earlier effort apparently was abandoned.

In the process of moving/verifying the above patch
to 2.6.31-, tripping over a
few build issues.  I believe this fix to be low-risk,
localized to hugetlbfs, and is simply reenabling
previously existing functionality.  Also as it is
still throttled by the security policy as described
above in comment #1, there is a userland resident
method to revert the feature.

The only minor unresolved issue associated with this
fix is a bit more concrete justification of
additional code needed to circumvent a !ptr dereference
in generic xattr operations, a clarification request
coming as a review comment from Stephen (TBD pending
fc12 kernel patch):

Stephen Smalley wrote:
> On Mon, 2009-08-17 at 16:23 -0400, john cooper wrote:
>> Stephen Smalley wrote:
>>> However:
>>> 1) You don't really need the listxattr/setxattr/getxattr handlers since
>>> the vfs_* functions will fall back automatically to calling the security
>>> hooks if the fs doesn't provide any.  So I think you only need the
>>> security_inode_init_security() call.
>> That's where I had started given the advertising preceding
>> fs/xattr.c:xattr_resolve_name(),  However the generic_*xattr()
>> operations kept tripping over a NULL inode->i_sb->s_xattr ptr
>> (perhaps a latent bug).  It appeared pulling in the additional
>> infrastructure was less risk in this context compared with
>> circumventing the NULL ptr deref in common code.
> I'm confused by that, as we get selinux xattr support for free for
> devpts by virtue of the vfs fallbacks.  We used to get it for free for
> tmpfs as well but they ultimately moved that down inside of tmpfs when
> they added POSIX ACL support.

Comment 5 Mark McLoughlin 2009-10-09 10:42:50 UTC
Okay, sounds like we'll have a patch suitable for F-12 soon

Comment 6 Bug Zapper 2010-03-15 12:45:08 UTC
This bug appears to have been reported against 'rawhide' during the Fedora 13 development cycle.
Changing version to '13'.

More information and reason for this action is here:

Comment 7 Bug Zapper 2011-06-02 17:51:46 UTC
This message is a reminder that Fedora 13 is nearing its end of life.
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Comment 8 Bug Zapper 2011-06-27 14:19:47 UTC
Fedora 13 changed to end-of-life (EOL) status on 2011-06-25. Fedora 13 is 
no longer maintained, which means that it will not receive any further 
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