Linux Kernel: unshare(2) CLONE_NEWNS bug leaves root/pwd on detached mounts
CVE-2026-43472 Published on May 8, 2026
unshare: fix unshare_fs() handling
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) ;-/
Products Associated with CVE-2026-43472
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Affected Versions
Linux:- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below 845bf3c6963a52096d0d3866e4a92db77a0c03d8 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below d3ffc8f13034af895531a02c30b1fe3a34b46432 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below d0d99f60538ddb4a62ccaac2168d8f448965f083 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below d7963d6997fea86a6def242ac36198b86655f912 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below aa9ebc084505fb26dd90f4d7a249045aad152043 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below 42e21e74061b0ebbd859839f81acf10efad02a27 is affected.
- Version 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and below 6c4b2243cb6c0755159bd567130d5e12e7b10d9f is affected.
- Version 5.10.253, <= 5.10.* is unaffected.
- Version 5.15.203, <= 5.15.* is unaffected.
- Version 6.1.167, <= 6.1.* is unaffected.
- Version 6.6.130, <= 6.6.* is unaffected.
- Version 6.12.78, <= 6.12.* is unaffected.
- Version 6.18.19, <= 6.18.* is unaffected.
- Version 6.19.9, <= 6.19.* is unaffected.
- Version 7.0, <= * is unaffected.