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 2C55A3BD649; Mon, 23 Mar 2026 16:13:35 +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=1774282416; cv=none; b=mNpQjyYKd2I6KQjNwBhSbd4VCJT9xSqwQv+MnGkO2YR3slK5ERn+0TZi7DwOV3Ux0aI+W+zdDKygNck9h3tNbGOmS3d4OEkPCQhSSqAmnnwlpxYUMA9qrYHmyJzTzsX0wXtxRStBcy0/gL2eNv58rS4bXkpCYyylvhQbD3e85oM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774282416; c=relaxed/simple; bh=52HkgNG9onzppc1vHfoQj758ZXPky5WL7oQvQwl58t8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MjbwvpriD1I0Ac261OPEnF+FV+d5w1s8xMp7aP4IU3GlFytS8+bpnup0mA8olSTgnZurVexQFQPqiwHkcZ1Ak1mCLLLBDmOobICbE660tuU+9ucCA8UOVQKjl5hTPmNaaVdn2Pea6D2hgoPYMebkALJ49KIAtlTWYYMR6jeSY+g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=BmSqOk+F; 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="BmSqOk+F" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 32C22C2BCB4; Mon, 23 Mar 2026 16:13:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774282415; bh=52HkgNG9onzppc1vHfoQj758ZXPky5WL7oQvQwl58t8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=BmSqOk+FWRKWqLtKEU4CmjqkHWhP1lnUqvXNYrPhEI9m+JJJvNbIfH2DWeE9g8BaX VhanMUqsB/2S8SGm52lT/QzVTiNO1itAYTpDYJ5fQGSHO5AEU4ekMDGvd9HsEPH3dJ v3pEmhVSNVHuWfT26mzBa8Y/3koBoA1m32WJDXFg= 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 6.1 149/481] unshare: fix unshare_fs() handling Date: Mon, 23 Mar 2026 14:42:11 +0100 Message-ID: <20260323134528.879638101@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134525.256603107@linuxfoundation.org> References: <20260323134525.256603107@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 6.1-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 c548538d3ade8..b8cf8891ffc7b 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -3193,7 +3193,7 @@ 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