ceph: fix up non-directory creation in SGID directories

commit fd84bfdddd169c219c3a637889a8b87f70a072c2 upstream.

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 0fa3ecd87848 ("Fix up non-directory creation in SGID directories")
[2]: commit 01ea173e103e ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 15 insertions, 3 deletions

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index e61018d9764e..e873c2ba7a7f 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -603,13 +603,25 @@ 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);