Re: [PATCH][RFC] bug in unshare(2) failure recovery

On Sat, Feb 07, 2026 at 08:25:24AM +0000, Al Viro wrote:
> On Fri, Feb 06, 2026 at 03:04:53PM -0500, Waiman Long wrote:
> 
> [summary of subthread: 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) ;-/
> 
> Signed-off-by: Al Viro <[email protected]>
> ---

I'll take that through my namespace tree as a fix.
Thanks!

Christian