From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 770393F12EF; Mon, 17 Aug 2026 14:00:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786975231; cv=none; b=RSwGkYUXy1o9u32D9sQX0QUFE0ZwRNw+7nQjQFRHsrfWx/RbVRhOj4A1stq8e64Os9ylS9Kf8KmTz9ckIMw803u8a+Pkm/SQ3WaCYamHLirQGA5jVBtHfzDqPbhvbOydYG9A22vuOH7NhE/k8xgIDKIaP1zfJw55h59SNVXo3sM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786975231; c=relaxed/simple; bh=UhAdvaj3t9Twht492jYNFaPD7FMUYSY0GKNYjCqpZ2A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=kJfH3lv7acwegLPs/eQ/QjGALrMkq1W3+1jXeCwRMTilWMkq+hXtitSdndBCN3xtuAlDn2DoFOJozvsNuB2LCVK7lUh1wHXmycAYStuC7o2MhfP9Wdf15lWIDDSY4O2V5BJDDhv+utRNRERGA5mCtstxUk6EipKvTcb1u+HbqH4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=zC8ni5g1; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="zC8ni5g1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C39731F00A3A; Mon, 17 Aug 2026 14:00:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1786975230; bh=+WG/R8yImWQNsYwO6YPWx2/RN2DdeNvGUgluc1u4K4A=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=zC8ni5g1G+BmW3jLB5rXwSrWO7DgXF5Yx8p8j8pyuMCbMv//5JEFvoMRr2CnbuOsm GihVmQ+Hq7h6QkZn/dCWMAKxZN4YiR/sBM7N/DYb+FRUuBuxi/nppKu2rcozDGHnoz lY7Fk4H3K454aJ28EtgrULdbE7e9lp5pNKUudCTY= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jan Kara , "Christian Brauner (Amutable)" Subject: [PATCH 6.18 208/250] binfmt_misc: dont warn when the mount is completed from another user namespace Date: Mon, 17 Aug 2026 15:32:49 +0200 Message-ID: <20260817132545.043748647@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260817132536.466235697@linuxfoundation.org> References: <20260817132536.466235697@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 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Brauner commit 79fdf39f1a31f88cb3833b6f8091fbf6acdca2c6 upstream. fsopen() records the caller's user namespace in fc->user_ns and hands back an ordinary file descriptor. Nothing ties the task that calls fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The fd is inherited across fork() and exec() and it can be passed over a unix socket. Completing a context from another user namespace is allowed on purpose. vfs_cmd_create() authorizes the create with mount_capable(), which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns. So an unprivileged task can reach the WARN_ON() in bm_fill_super(): create a user and a mount namespace in a child, call fsopen("binfmt_misc") there, send the fscontext fd to the parent and let the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain unshare(1) and no capability is needed anywhere: WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc] CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn Call Trace: get_tree_keyed+0x7d/0xb0 bm_get_tree+0x34/0x90 [binfmt_misc] vfs_get_tree+0x2a/0x100 vfs_cmd_create+0x60/0xf0 __do_sys_fsconfig+0x4b2/0x500 The child needs the mount namespace because fsopen() itself gates on may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount namespace. fsconfig() doesn't repeat that check. It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be raised in a loop to taint the kernel and flood the log, and it panics a kernel booted with panic_on_warn. Keep refusing the mount and stop warning about it. Nothing in bm_fill_super() depends on the two namespaces matching, it derives everything from sb->s_user_ns. Fixes: 21ca59b365c0 ("binfmt_misc: enable sandboxed mounts") Cc: stable@vger.kernel.org # v6.7+ Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-2-4e987911a39a@kernel.org Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman --- fs/binfmt_misc.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/fs/binfmt_misc.c +++ b/fs/binfmt_misc.c @@ -953,7 +953,8 @@ static int bm_fill_super(struct super_bl /* last one */ {""} }; - if (WARN_ON(user_ns != current_user_ns())) + /* The fscontext fd may have been passed to another user namespace. */ + if (user_ns != current_user_ns()) return -EINVAL; /* Never exec off this instance and never let anything stack on it. */