ceph: fix up non-directory creation in SGID directories

Ceph always inherits the SGID bit if it is set on the parent inode,
while the generic inode_init_owner does not do this in a few cases where
it can create a possible security problem (cf. [1]).

Update ceph to strip the SGID bit just as inode_init_owner would.

This bug was detected by the mapped mount testsuite in [3]. The
testsuite tests all core VFS functionality and semantics with and
without mapped mounts. That is to say it functions as a generic VFS
testsuite in addition to a mapped mount testsuite. While working on
mapped mount support for ceph, SIGD inheritance was the only failing
test for ceph after the port.

The same bug was detected by the mapped mount testsuite in XFS in
January 2021 (cf. [2]).

[1]: commit ea228d760707 ("Fix up non-directory creation in SGID directories")
[2]: commit 1db3fee044c4 ("xfs: fix up non-directory creation in SGID directories")
[3]: https://git.kernel.org/fs/xfs/xfstests-dev.git

Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index b24442e27e4e0422fc352d2e40a53b3643e9f40c..c138e8126286cfe52996f982071003fbc95243a6 100644 (file)
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -605,13 +605,25 @@ static int ceph_finish_async_create(struct inode *dir, struct dentry *dentry,
        in.cap.realm = cpu_to_le64(ci->i_snap_realm->ino);
        in.cap.flags = CEPH_CAP_FLAG_AUTH;
        in.ctime = in.mtime = in.atime = iinfo.btime;
-       in.mode = cpu_to_le32((u32)mode);
        in.truncate_seq = cpu_to_le32(1);
        in.truncate_size = cpu_to_le64(-1ULL);
        in.xattr_version = cpu_to_le64(1);
        in.uid = cpu_to_le32(from_kuid(&init_user_ns, current_fsuid()));
-       in.gid = cpu_to_le32(from_kgid(&init_user_ns, dir->i_mode & S_ISGID ?
-                               dir->i_gid : current_fsgid()));
+       if (dir->i_mode & S_ISGID) {
+               in.gid = cpu_to_le32(from_kgid(&init_user_ns, dir->i_gid));
+
+               /* Directories always inherit the setgid bit. */
+               if (S_ISDIR(mode))
+                       mode |= S_ISGID;
+               else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
+                        !in_group_p(dir->i_gid) &&
+                        !capable_wrt_inode_uidgid(&init_user_ns, dir, CAP_FSETID))
+                       mode &= ~S_ISGID;
+       } else {
+               in.gid = cpu_to_le32(from_kgid(&init_user_ns, current_fsgid()));
+       }
+       in.mode = cpu_to_le32((u32)mode);
+
        in.nlink = cpu_to_le32(1);
        in.max_size = cpu_to_le64(lo->stripe_unit);