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 AE47D30C360; Mon, 13 Apr 2026 16:23:29 +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=1776097409; cv=none; b=qLJxHPYnVdcUwtcwPrSrHHyvfM7LmNhm6thhBUfrQiGwKh8znAZdRs0ZiVZC+Dt0LU9tzvH70mPzdke1MsV59imSWAjKUXbq/exktscWV9WoQsvEj9pSB2asi6QJMgTys8ZwPWZ7rTBjyw3yztH6T4QnmDjHKbfZUXsPRccV3Cw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776097409; c=relaxed/simple; bh=srRQag1OvRYeqXwZHJ/8UvAheXYcBrkvE4ckWVg/Q/0=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=myP9XG2R37Xb1DJ7lDypDPYowj7yvCT2yREkU1tV0piYkLkeIz5EbmLVwQMrH86n1sMiJPyTUo1mMll+V+yJis2GjCeCm/+ddz3zW0MWfZblkwnqbLUG8wAHcwSoPx+JCogzAkpVse303S9Q/mhxwPj1zHN9OLB4ooLild77GuM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=VXJsGBrN; 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="VXJsGBrN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EC83EC2BCB6; Mon, 13 Apr 2026 16:23:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1776097409; bh=srRQag1OvRYeqXwZHJ/8UvAheXYcBrkvE4ckWVg/Q/0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=VXJsGBrN+05BTUl+zEFl22Bi7ef4NEEP5crtMsD+Ae8x+4dyggsa9EKftIOhy6u7O JeBRjXEqqmFCtS06D3uxMwAeHTeNo1LUZX2oh1tzJyzXIou7G/xV966toBLFjNmAVC F8BLQ1dcU8k8SYM3PYMRw4ref/E8xe0OVLab92+I= 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 5.15 093/570] unshare: fix unshare_fs() handling Date: Mon, 13 Apr 2026 17:53:44 +0200 Message-ID: <20260413155833.927549298@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260413155830.386096114@linuxfoundation.org> References: <20260413155830.386096114@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-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 2c99d39e2bc08..e1b291e5e1038 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -3086,7 +3086,7 @@ 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