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

Official 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) ;-/

Affected packages & fixed versions

Affected products (as reported)

References

• 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