From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 28121377EA5; Tue, 17 Mar 2026 16:37:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773765455; cv=none; b=k5Pu5msBuAg1RnawMJWwUsazR6syLhJTdzt7SCZ+aUnoRDjIF/Rj2rlh8pamDJijgUBKeaGyBC4gv/y1tjZ99fbJJJLwf0hDP7XhlxbHiG87JfTv4TgICl1pHH0LqDH9tTyaTTr8n8yTw67r1ClJu+LSkyCEzpU03VOdywPkOkA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773765455; c=relaxed/simple; bh=ZInbshZsplJwNiIWWNFiYslJFmp/Fkpp2eSS66CeNw8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jm3wh9G7Z5GFVG1zZlFhLh96zzu22tBG7r4Nlkz/TcDs1kKR7jXzsLjsWXtPDo8z+IcQd5nGVChzvSxNu19vmkHftW1R6dHYNlXUJaQ+GDCiqGCxjiEpVI3bpXbl4+B4rLHmpg7YjPaDBlP56JagHQkzPfzNCF4mkPVBtFPrdkA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=XMD2qZ2J; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="XMD2qZ2J" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 10605C4CEF7; Tue, 17 Mar 2026 16:37:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1773765454; bh=ZInbshZsplJwNiIWWNFiYslJFmp/Fkpp2eSS66CeNw8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=XMD2qZ2Jrh+/gc4aYwEMT52N0P1z59NPNCrkxkOS4wvznoeCv3eE0PD/i4P21MJUS 3SxIi6OqAex7ytllRuxbjIKQMp5hM0Hl3DrX4JAtmZ7O9QMA9/jkuIOfTjYKMxiLsL 7+XhwZYbVW4jl3QBHXE7rK7pK+3G/ZORly8PM4GI= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Al Viro , Waiman Long , Christian Brauner , Sasha Levin Subject: [PATCH 6.19 015/378] unshare: fix unshare_fs() handling Date: Tue, 17 Mar 2026 17:29:32 +0100 Message-ID: <20260317163007.535445297@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260317163006.959177102@linuxfoundation.org> References: <20260317163006.959177102@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.19-stable review patch. If anyone has any objections, please let me know. ------------------ From: Al Viro [ 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 Link: https://patch.msgid.link/20260207082524.GE3183987@ZenIV Tested-by: Waiman Long Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin --- kernel/fork.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/fork.c b/kernel/fork.c index b1f3915d5f8ec..68ccbaea7398a 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); -- 2.51.0