[PATCH 6.6 201/567] unshare: fix unshare_fs() handling

Greg Kroah-Hartman
2026-03-23T15:01:37Z

6.6-stable review patch. If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@zeniv.linux.org.uk>

[ Upstream commit 6c4b2243cb6c0755159bd567130d5e12e7b10d9f ]

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 <viro@zeniv.linux.org.uk>
Link: https://patch.msgid.link/20260207082524.GE3183987@ZenIV
Tested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/fork.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 2141ebb2ef92a..ce6f6e1e39057 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3350,7 +3350,7 @@ 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);
--
2.51.0