CVE-2026-89649: ceph: bound xattr value length in __build_xattrs()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

ceph: bound xattr value length in __build_xattrs()

__build_xattrs() decodes the MDS-supplied xattr blob one attribute at a
time. For each attribute it reads a 32-bit name length, advances past the
name bytes, reads a 32-bit value length, records the value pointer, and
advances past the value bytes. The two length fields are read with
ceph_decode_32_safe(), but the value bytes themselves are advanced over
with a bare "p += len" and no ceph_decode_need() check that "len" bytes
remain in the blob.

For every attribute except the last, the next iteration's
ceph_decode_32_safe() on the following name length implicitly verifies
that the previous value did not run past the blob end. The final
attribute has no successor, so its decoded value length is never checked
against the blob bounds. A malicious or compromised metadata server can
set the last attribute's value length larger than the bytes actually
present in the blob.

The blob is a dedicated kvmalloc() allocation sized to the wire length
(ceph_buffer_new() in ceph_fill_inode()). __set_xattr() records the
oversized length in xattr->val_len verbatim, and a later getxattr(2) runs
memcpy(value, xattr->val, xattr->val_len) into a user-supplied buffer,
copying bytes past the end of the allocation back to user space.

Impact: a malicious metadata server discloses adjacent kernel heap bytes
to a local user via getxattr(2) on a CephFS file. Add the missing
ceph_decode_need() so an out-of-bounds value length on the final
attribute fails the decode and returns -EIO instead of being stored.

The Linux kernel CVE team has assigned CVE-2026-89649 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.34 with commit 355da1eb7a1f91c276b991764e951bbcd8047599 and fixed in 6.12.109 with commit 38be2f3c41ecaba58bb92dc16603e09ba9fd64f1
	Issue introduced in 2.6.34 with commit 355da1eb7a1f91c276b991764e951bbcd8047599 and fixed in 6.18.50 with commit 1dd356310b1660215f9a3626580bf8b363ef981e
	Issue introduced in 2.6.34 with commit 355da1eb7a1f91c276b991764e951bbcd8047599 and fixed in 7.2.4 with commit b12b3320c81fd72ab36388fb2445fec9c3038f10
	Issue introduced in 2.6.34 with commit 355da1eb7a1f91c276b991764e951bbcd8047599 and fixed in 7.3-rc1 with commit 68d541754d6cd3bb98d1fd8314f57e5eb533557d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89649
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/ceph/xattr.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/38be2f3c41ecaba58bb92dc16603e09ba9fd64f1
	https://git.kernel.org/stable/c/1dd356310b1660215f9a3626580bf8b363ef981e
	https://git.kernel.org/stable/c/b12b3320c81fd72ab36388fb2445fec9c3038f10
	https://git.kernel.org/stable/c/68d541754d6cd3bb98d1fd8314f57e5eb533557d