linux-kernel - [PATCH][RFC] bug in unshare(2) failure recovery

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20260207082524.GE3183987@ZenIV>
Date: Sat, 7 Feb 2026 08:25:24 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	linux-kernel@...r.kernel.org, audit@...r.kernel.org,
	Richard Guy Briggs <rgb@...hat.com>,
	Ricardo Robaina <rrobaina@...hat.com>,
	Waiman Long <llong@...hat.com>
Subject: [PATCH][RFC] bug in unshare(2) failure recovery

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 <viro@...iv.linux.org.uk>
---
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);