CVE-2026-98990: namespace: check a submount for references right before unmounting it

CVE-2026-98990: namespace: check a submount for references right before unmounting it

[Greg Kroah-Hartman]

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

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

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

namespace: check a submount for references right before unmounting it

shrink_submounts() and mark_mounts_for_expiry() first collect all the
mounts they are allowed to unmount and then unmount them.

Whether a mount is busy is decided by propagate_mount_busy() on the
tree. But unmounting the first mount changes the tree that the second
one propagates into. propagate_umount()
moves a surviving overmount off a copy it unmounts and mounts it where
the copy was mounted. If that's where the propagated copy of the second
victim is looked up the overmount becomes a candidate of the second
umount_tree(). It's childless and so trim_one() commits it without
looking at its reference count.

So a synchronous umount pulls out a busy mount that is in use somewhere
else and marks it MNT_SYNC_UMOUNT while it has users:

  umount2(/tmp/plshrink/p1, 0) = 0 errno 0
  cwd is (unreachable)

The same two umounts requested one after the other fail with EBUSY.
This needs a mount with MNT_SHRINKABLE, i.e., automounted submounts of
NFS, AFS and CIFS or the tracefs mount below debugfs. Bind mounts
inherit the flag.

Check each mount right before it is unmounted under the same hold of
namespace_sem and mount_lock as the umount itself. Make sure that the
algorithm stays linear.

The Linux kernel CVE team has assigned CVE-2026-98990 to this issue.


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

	Issue introduced in 4.11 with commit 1064f874abc0d05eeed8993815f584d847b72486 and fixed in 6.6.160 with commit f6394281834492c6516a405a348b545ae698a331
	Issue introduced in 4.11 with commit 1064f874abc0d05eeed8993815f584d847b72486 and fixed in 6.12.113 with commit 6e4012e571867868b545c91ca9d6095fdb709907
	Issue introduced in 4.11 with commit 1064f874abc0d05eeed8993815f584d847b72486 and fixed in 6.18.57 with commit d73f04e92c1aab78d96ac6cadb234c8a3a373d22
	Issue introduced in 4.11 with commit 1064f874abc0d05eeed8993815f584d847b72486 and fixed in 7.2.11 with commit 18902a23ef36f83d32bafc58ae2484e6bb4bb8ef
	Issue introduced in 4.11 with commit 1064f874abc0d05eeed8993815f584d847b72486 and fixed in 7.3 with commit 32bf92416614a1e3ff3524fc6b780259c7aad3ef
	Issue introduced in 4.1.40 with commit 5e6ef5ef6eaf690faad3a7c6c665ebf45d7b6ccf
	Issue introduced in 4.4.54 with commit 839d42687dfce0ed0ea2c6bd8d707cc0e276fbe7
	Issue introduced in 4.9.15 with commit 808e83e5add13152bd4a88346487de68395c136b
	Issue introduced in 4.10.3 with commit 6de9d08a9718ee9c12ba6343c50141c2d633e3dc

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-2026-98990
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/namespace.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/f6394281834492c6516a405a348b545ae698a331
	https://git.kernel.org/stable/c/6e4012e571867868b545c91ca9d6095fdb709907
	https://git.kernel.org/stable/c/d73f04e92c1aab78d96ac6cadb234c8a3a373d22
	https://git.kernel.org/stable/c/18902a23ef36f83d32bafc58ae2484e6bb4bb8ef
	https://git.kernel.org/stable/c/32bf92416614a1e3ff3524fc6b780259c7aad3ef