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 15EE43515F1; Tue, 13 Oct 2026 12:34:50 +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=1791894891; cv=none; b=t9rjp5IKpl1bwtbe6Hh6p/DzNcRAL6XLNaXad2W9OapNDHJzk3FLr++ZSHXFGumfeLY0JJ4a0pyaSSh4fItLX+utZUBFGpDgm1QMH8yGvlUVKVu+zuk+ZKAQpWrGApoLy7EpM8JG37wzYE13dFm/szKTUvsdvL6/YyRAELJnOns= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791894891; c=relaxed/simple; bh=2Reo4TYkTIJi97sVd8aWAbloQnDxtMBAF0G9x+CEqng=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=bqIUeSiffQa7qyzI9bBzWh3CJGmoYsELRlccPqFMcjk35KWTxm7xpLAiCRtXz2gSeYC1AGPLmGij21NW76FYTW4hWZ3wb6NofjVzzYDb4JI3aZ1XhxSec+OesXeDtwSsPqspAh2AokXa80iDdztdp4mWPjAKgou29bKZnW1WKT0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=WZUthWli; 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="WZUthWli" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 728D51F000FF; Tue, 13 Oct 2026 12:34:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1791894890; bh=jbijNf+ecrtHSylTn/HDtlLUsqqbGT733If50C4gJcY=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=WZUthWligQljfEw0aZ8OsgFeyTnXWHMOMeoy1tENW9C48lwsCrLibglO3pzxx8MA0 5lMEo2eXP+kbE8cIiGZiwnrceK+lHgHa56dpLAhLa3y5aHj2g+cqpPxBojfN5Qju28 U7e+7kn7ueh8XkrxakQW4ffCEjtHa4T6+gRmkgZU= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jiangshan Yi , Namjae Jeon , Paulo Alcantara , Bjoern Doebel , Sasha Levin Subject: [PATCH 6.12 037/497] smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions Date: Tue, 13 Oct 2026 13:33:24 +0200 Message-ID: <20261013112908.770236855@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261013112907.864180619@linuxfoundation.org> References: <20261013112907.864180619@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 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jiangshan Yi commit b8e5dc4f95e5484159b343903f302eb6d783f2e6 upstream. When a file has the setuid or setgid bit set and is written to, the VFS strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID together with an ATTR_MODE carrying the already-cleared mode. Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped ATTR_MODE in that case: /* skip mode change if it's just for clearing setuid/setgid */ if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) attrs->ia_valid &= ~ATTR_MODE; This is fine for the default mount, where the mode is only emulated via the DOS read-only attribute and cannot represent the setuid/setgid bits anyway. However, with the "cifsacl" or "modefromsid" mount options the mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with the SMB3.1.1 POSIX extensions the mode is sent to the server directly, and with the SMB1 Unix extensions (cifs_setattr_unix) the mode is sent via CIFSSMBUnixSetPathInfo(). In all those cases dropping ATTR_MODE means the cleared mode is never pushed to the server, so the setuid/setgid bit survives the write. This is a security issue: on local filesystems the setuid bit is stripped when a file is written, but over these cifs.ko mounts the bit persists on the server, potentially allowing an unexpected privilege escalation on subsequent execution. Fix this in two places: 1. cifs_setattr_nounix(): only take the "skip mode change" shortcut when the mode is emulated via the DOS read-only attribute (i.e. neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are in effect), so that the cleared mode is propagated to the server in the ACL / POSIX cases. 2. cifs_setattr_unix(): this function is only called when Unix extensions are in effect, so the mode is always stored on the server. Remove the shortcut entirely so that the cleared mode is always pushed. Fixes: d32c4f2626ac ("CIFS: ignore mode change if it's just for clearing setuid/setgid bits") Cc: stable@vger.kernel.org Signed-off-by: Jiangshan Yi Signed-off-by: Namjae Jeon Signed-off-by: Paulo Alcantara [ Backport note for 6.12: no sbflags local exists here, so test cifs_sb->mnt_cifs_flags directly (same flag bits); uses existing "bool posix". ] Signed-off-by: Bjoern Doebel Assisted-by: opencode:kimi-k3 Signed-off-by: Sasha Levin --- fs/smb/client/inode.c | 30 +++++++++++++++++++++++++----- 1 file changed, 25 insertions(+), 5 deletions(-) diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c index b67b316538b83..7c4828580076c 100644 --- a/fs/smb/client/inode.c +++ b/fs/smb/client/inode.c @@ -3117,9 +3117,13 @@ cifs_setattr_unix(struct dentry *direntry, struct iattr *attrs) goto out; } - /* skip mode change if it's just for clearing setuid/setgid */ - if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) - attrs->ia_valid &= ~ATTR_MODE; + /* + * This function is only called when Unix extensions are in effect, + * so the mode is always sent to and stored on the server. Do not + * skip the mode change when clearing setuid/setgid bits: dropping + * ATTR_MODE here would leave those bits set on the server after a + * write, which is a security issue. + */ args = kmalloc(sizeof(*args), GFP_KERNEL); if (args == NULL) { @@ -3307,8 +3311,24 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs) if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_SET_UID)) attrs->ia_valid &= ~(ATTR_UID | ATTR_GID); - /* skip mode change if it's just for clearing setuid/setgid */ - if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) + /* + * Skip the mode change if it is only being done to clear the + * setuid/setgid bits *and* the mode is emulated via the DOS + * read-only attribute (the default, non-ACL case), which cannot + * represent the setuid/setgid bits anyway. + * + * When the mode is instead stored on the server - i.e. with the + * cifsacl or modefromsid mount options (via an ACL) or with the + * SMB3.1.1 POSIX extensions - the cleared mode must be pushed to + * the server. Dropping ATTR_MODE here would leave the setuid/ + * setgid bit set on the server after a write, which is a security + * issue (the bits are not stripped as they are on local + * filesystems). + */ + if ((attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) && + !((cifs_sb->mnt_cifs_flags & + (CIFS_MOUNT_CIFS_ACL | CIFS_MOUNT_MODE_FROM_SID)) || + posix)) attrs->ia_valid &= ~ATTR_MODE; if (attrs->ia_valid & ATTR_MODE) { -- 2.53.0