CVE-2026-74484: binfmt_misc: don't let an 'F' entry pin its own instance

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

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

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

binfmt_misc: don't let an 'F' entry pin its own instance

An entry registered with 'F' opens its interpreter at registration time
and holds that file until the entry is freed. Any entry nobody removes
by hand only gets closed once the binfmt_misc superblock is shut down.
If the interpreter lives on a mount that keeps that superblock alive the
two pin each other:

    binfmt_misc sb -> inode -> entry -> interp_file -> vfsmount -> binfmt_misc sb

TL;DR the file is never closed. Once the mount namespace is gone there
is nothing left to unregister through either.

There are two ways to trigger this bug:

- Point the interpreter at the instance itself. Its files are regular
  files owned by the mounter and both bm_get_inode() and
  simple_fill_super() leave i_op at empty_iops. So notify_change() falls
  back to simple_setattr() and chmod +x works. We never set SB_I_NOEXEC
  and so open_exec() accepts it.

- Use the instance as an overlayfs lower layer. The overlay superblock
  holds a clone_private_mount() of every layer until it is destroyed and
  that clone is in no namespace. So umount_tree() never reaches it.

That's a DoS. And it isn't only the superblock that leaks. It pins the
user namespace it was mounted in, so every iteration permanently eats
one of the caller's user namespace charges.

So let's just do the sane thing. SB_I_NOEXEC makes open_exec() fail on
the instance's own files and s_stack_depth makes overlayfs reject the
layer before it ever takes a clone. That also covers the ecryptfs and
fuse passthrough variants. What 'F' promises is unchanged.

The stable tag is narrower than the Fixes tags on purpose. Before
sandboxed mounts this needed global root against the single instance
everyone shares, and the change doesn't apply to those trees anyway.

Note that SB_I_NODEV is implicitly raised for userns mounts but raise it
explicitly here as well.

The Linux kernel CVE team has assigned CVE-2026-74484 to this issue.


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

	Issue introduced in 4.8 with commit 948b701a607f123df92ed29084413e5dd8cda2ed and fixed in 6.12.103 with commit 1cc2decee06acc939337304e9b3f737fd5d8c4bd
	Issue introduced in 4.8 with commit 948b701a607f123df92ed29084413e5dd8cda2ed and fixed in 6.18.44 with commit 4dad8ca637d44d5a6d5c23fa80c9e2e455198c2b
	Issue introduced in 4.8 with commit 948b701a607f123df92ed29084413e5dd8cda2ed and fixed in 7.1.8 with commit 098e92fe0f1bde5af99c8cf504f13f01ef139f85
	Issue introduced in 4.8 with commit 948b701a607f123df92ed29084413e5dd8cda2ed and fixed in 7.2-rc6 with commit 79055d82772b9584f259b747fe40ff56a076678d

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-2026-74484
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/binfmt_misc.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/1cc2decee06acc939337304e9b3f737fd5d8c4bd
	https://git.kernel.org/stable/c/4dad8ca637d44d5a6d5c23fa80c9e2e455198c2b
	https://git.kernel.org/stable/c/098e92fe0f1bde5af99c8cf504f13f01ef139f85
	https://git.kernel.org/stable/c/79055d82772b9584f259b747fe40ff56a076678d