CVE-2026-74619: ovl: don't warn when the mount is completed from another user namespace

From: Greg Kroah-Hartman <[email protected]>

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

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

ovl: don't warn when the mount is completed from another user namespace

fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor. Nothing ties the task that calls
fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The
fd is inherited across fork() and exec() and it can be passed over a
unix socket.

Completing a context from another user namespace is allowed on purpose.
vfs_cmd_create() authorizes the create with mount_capable(), which for
FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that
succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns.
So an unprivileged task can reach the WARN_ON() in ovl_fill_super():
create a user and a mount namespace in a child, call fsopen("overlay")
there, send the fscontext fd to the parent and let the parent issue
FSCONFIG_CMD_CREATE. Both namespaces come from a plain unshare(1) and no
capability is needed anywhere:

  WARNING: fs/overlayfs/super.c:1551 at ovl_fill_super+0x7b9/0x1e20 [overlay]
  CPU: 3 UID: 1000 PID: 3243376 Comm: fswarn
  Call Trace:
   get_tree_nodev+0x71/0xa0
   ovl_get_tree+0x15/0x20 [overlay]
   vfs_get_tree+0x2a/0x100
   vfs_cmd_create+0x60/0xf0
   __do_sys_fsconfig+0x4b2/0x500

The child needs the mount namespace because fsopen() itself gates on
may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning
the caller's mount namespace. fsconfig() doesn't repeat that check.

It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be
raised in a loop to taint the kernel and flood the log, and it panics a
kernel booted with panic_on_warn.

Keep refusing the mount and stop warning about it. ovl_parse_param()
already spells a user namespace check this way for Opt_override_creds.

The Linux kernel CVE team has assigned CVE-2026-74619 to this issue.


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

	Issue introduced in 6.5 with commit 1784fbc2ed9c888ea4e895f30a53207ed7ee8208 and fixed in 6.6.152 with commit 513478092966dc9818d96dd2b3ed613fd2f6e30e
	Issue introduced in 6.5 with commit 1784fbc2ed9c888ea4e895f30a53207ed7ee8208 and fixed in 6.12.104 with commit 494346f2aab2489d379d43ff614aea447cf4e94d
	Issue introduced in 6.5 with commit 1784fbc2ed9c888ea4e895f30a53207ed7ee8208 and fixed in 6.18.45 with commit be161fa31e3e9cc828a3c1bd935edca461e8a7a1
	Issue introduced in 6.5 with commit 1784fbc2ed9c888ea4e895f30a53207ed7ee8208 and fixed in 7.1.9 with commit 42d99fcd8006007e2f708bede6789f37f3910b30
	Issue introduced in 6.5 with commit 1784fbc2ed9c888ea4e895f30a53207ed7ee8208 and fixed in 7.2 with commit 63981fc786daaa626cb14d9be1406f674d79f98f

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-2026-74619
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/overlayfs/super.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/513478092966dc9818d96dd2b3ed613fd2f6e30e
	https://git.kernel.org/stable/c/494346f2aab2489d379d43ff614aea447cf4e94d
	https://git.kernel.org/stable/c/be161fa31e3e9cc828a3c1bd935edca461e8a7a1
	https://git.kernel.org/stable/c/42d99fcd8006007e2f708bede6789f37f3910b30
	https://git.kernel.org/stable/c/63981fc786daaa626cb14d9be1406f674d79f98f