From: Jiangshan Yi <yijiangshan@kylinos.cn> stable inclusion from stable-v6.18.51 commit b10015807e4c628095d1d1d1c9307efc8cdd9e1b category: bugfix bugzilla: https://atomgit.com/src-openeuler/kernel/issues/18923 CVE: CVE-2026-89638 Reference: https://web.git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/... -------------------------------- [ Upstream commit b8e5dc4f95e5484159b343903f302eb6d783f2e6 ] 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 <yijiangshan@kylinos.cn> Signed-off-by: Namjae Jeon <linkinjeon@kernel.org> Signed-off-by: Paulo Alcantara <pc@manguebit.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Conflicts: fs/cifs/inode.c fs/smb/client/inode.c [1. Commit b8e5dc4f95e5 touches fs/smb/client/inode.c, which this tree still has as fs/cifs/inode.c. The new check used the upstream-local sbflags variable, which does not exist here, so use cifs_sb->mnt_cifs_flags instead. 2. Commit 412094a8fb07 ("smb3: add new mount option to retrieve mode from special ACE") introduce CIFS_MOUNT_MODE_FROM_SID, this version has not merged.] Signed-off-by: Zizhi Wo <wozizhi@huawei.com> Signed-off-by: Zizhi Wo <wozizhi@huawei.com> --- fs/cifs/inode.c | 29 ++++++++++++++++++++++++----- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/fs/cifs/inode.c b/fs/cifs/inode.c index e5d0066f6125..3865ca0ce04a 100644 --- a/fs/cifs/inode.c +++ b/fs/cifs/inode.c @@ -2290,13 +2290,17 @@ cifs_setattr_unix(struct dentry *direntry, struct iattr *attrs) rc = cifs_set_file_size(inode, attrs, xid, full_path); if (rc != 0) 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) { rc = -ENOMEM; goto out; @@ -2461,12 +2465,27 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs) } else #endif /* CONFIG_CIFS_ACL */ 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_sb_master_tcon(cifs_sb)->posix_extensions)) attrs->ia_valid &= ~ATTR_MODE; if (attrs->ia_valid & ATTR_MODE) { mode = attrs->ia_mode; rc = 0; -- 2.52.0