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

On 2/7/26 3:25 AM, 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]>
> ---
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b1f3915d5f8e..68ccbaea7398 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -3082,7 +3082,7 @@ static int unshare_fs(unsigned long unshare_flags, struct fs_struct **new_fsp)
>   		return 0;
>   
>   	/* don't need lock here; in the worst case we'll do useless copy */
> -	if (fs->users == 1)
> +	if (!(unshare_flags & CLONE_NEWNS) && fs->users == 1)
>   		return 0;
>   
>   	*new_fsp = copy_fs_struct(fs);
>
After booting up a vanilla 6.19.0-rc8 kernel, I found that copy_mnt_ns()
was called 13 times during the bootup process with current->fs passed
down to it. After applying this patch, the count dropped to 0.

Tested-by: Waiman Long <[email protected]>