CVE-2026-43472: unshare: fix unshare_fs() handling

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

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

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

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

unshare: fix unshare_fs() handling

There's an unpleasant corner case in unshare(2), when we have a
CLONE_NEWNS in flags and current->fs hadn't been shared at all; in that
case copy_mnt_ns() gets passed current->fs instead of a private copy,
which causes interesting warts in proof of correctness]

> I guess if private means fs->users == 1, the condition could still be true.

Unfortunately, it's worse than just a convoluted proof of correctness.
Consider the case when we have CLONE_NEWCGROUP in addition to CLONE_NEWNS
(and current->fs->users == 1).

We pass current->fs to copy_mnt_ns(), all right.  Suppose it succeeds and
flips current->fs->{pwd,root} to corresponding locations in the new namespace.
Now we proceed to copy_cgroup_ns(), which fails (e.g. with -ENOMEM).
We call put_mnt_ns() on the namespace created by copy_mnt_ns(), it's
destroyed and its mount tree is dissolved, but...  current->fs->root and
current->fs->pwd are both left pointing to now detached mounts.

They are pinning those, so it's not a UAF, but it leaves the calling
process with unshare(2) failing with -ENOMEM _and_ leaving it with
pwd and root on detached isolated mounts.  The last part is clearly a bug.

There is other fun related to that mess (races with pivot_root(), including
the one between pivot_root() and fork(), of all things), but this one
is easy to isolate and fix - treat CLONE_NEWNS as "allocate a new
fs_struct even if it hadn't been shared in the first place".  Sure, we could
go for something like "if both CLONE_NEWNS *and* one of the things that might
end up failing after copy_mnt_ns() call in create_new_namespaces() are set,
force allocation of new fs_struct", but let's keep it simple - the cost
of copy_fs_struct() is trivial.

Another benefit is that copy_mnt_ns() with CLONE_NEWNS *always* gets
a freshly allocated fs_struct, yet to be attached to anything.  That
seriously simplifies the analysis...

FWIW, that bug had been there since the introduction of unshare(2) ;-/

The Linux kernel CVE team has assigned CVE-2026-43472 to this issue.


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

	Fixed in 5.10.253 with commit 845bf3c6963a52096d0d3866e4a92db77a0c03d8
	Fixed in 5.15.203 with commit d3ffc8f13034af895531a02c30b1fe3a34b46432
	Fixed in 6.1.167 with commit d0d99f60538ddb4a62ccaac2168d8f448965f083
	Fixed in 6.6.130 with commit d7963d6997fea86a6def242ac36198b86655f912
	Fixed in 6.12.78 with commit aa9ebc084505fb26dd90f4d7a249045aad152043
	Fixed in 6.18.19 with commit af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
	Fixed in 6.19.9 with commit 42e21e74061b0ebbd859839f81acf10efad02a27
	Fixed in 7.0 with commit 6c4b2243cb6c0755159bd567130d5e12e7b10d9f

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-2026-43472
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:
	kernel/fork.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/845bf3c6963a52096d0d3866e4a92db77a0c03d8
	https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432
	https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083
	https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912
	https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043
	https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
	https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27
	https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f