CVE-2026-89766: pidfd: hold exec_update_lock around namespace ioctl

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

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

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

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

pidfd: hold exec_update_lock around namespace ioctl

The PIDFD_GET_*_NAMESPACE ioctls in pidfd_ioctl() perform a filesystem
credentials ptrace access check before handing out a namespace file
descriptor.  The accompanying comment states that the code "mirrors nsfs
behavior", but, unlike the corresponding procfs paths, it does so without
holding the target task's exec_update_lock.

proc_ns_get_link() and proc_ns_readlink() both take exec_update_lock for
reading around the ptrace check and the namespace lookup, so that the
credentials used for the access decision match those of the task when its
namespace is read.  Without it, a caller can pass the check against the
target's old credentials and then read the namespace after the target has
execve()'d a setuid binary and committed new credentials -- accessing
namespace information it should have been denied.

Hold exec_update_lock for reading around the ptrace check and the
namespace lookup so that pidfd truly mirrors nsfs behavior, as the comment
already claims.  open_namespace() itself runs outside the lock: once a
namespace reference is obtained it carries its own refcount and is opened
with the caller's own credentials, so a concurrent execve() on the target
can no longer affect the outcome.

The Linux kernel CVE team has assigned CVE-2026-89766 to this issue.


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

	Issue introduced in 6.11 with commit 5b08bd408534bfb3a7cf5778da5b27d4e4fffe12 and fixed in 6.18.51 with commit cf24ce48d60225fdbee68ed2058763a1fd172ff3
	Issue introduced in 6.11 with commit 5b08bd408534bfb3a7cf5778da5b27d4e4fffe12 and fixed in 7.2.4 with commit e89318aa8629969e86548b05e7c19283af3f0efb
	Issue introduced in 6.11 with commit 5b08bd408534bfb3a7cf5778da5b27d4e4fffe12 and fixed in 7.3-rc1 with commit 9688a46802939da28f00cb40e8129615d5d4af39

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-2026-89766
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/pidfs.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/cf24ce48d60225fdbee68ed2058763a1fd172ff3
	https://git.kernel.org/stable/c/e89318aa8629969e86548b05e7c19283af3f0efb
	https://git.kernel.org/stable/c/9688a46802939da28f00cb40e8129615d5d4af39