[PATCH v5.10] ceph: fix up non-directory creation in SGID directories

[Christian Brauner <brauner@kernel.org>]

From: Christian Brauner <christian.brauner@ubuntu.com>

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 (adapted to v5.10)
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
---
 fs/ceph/file.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 8e6855e7ed83..8ed881fd7440 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(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);
 
-- 
2.30.2