[PATCH v2 10/28] sys:__sys_setgid(): handle fsid mappings

[Christian Brauner <christian.brauner@xxxxxxxxxx>]

Switch setgid() to lookup fsids in the fsid mappings. If no fsid mappings are
setup the behavior is unchanged, i.e. fsids are looked up in the id mappings.
The kfsid to cleanly handle userns visible filesystem is set as before.

We require that a user must have a valid fsid mapping for the target id. This
is consistent with how the setid calls work today without fsid mappings.

Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
/* v2 */
- Christian Brauner <christian.brauner@xxxxxxxxxx>:
  - set kfsid which is used when dealing with proc permission checking
---
 kernel/sys.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index a8eefd748327..aa379fb5e93b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -416,24 +416,31 @@ long __sys_setgid(gid_t gid)
 	const struct cred *old;
 	struct cred *new;
 	int retval;
-	kgid_t kgid;
+	kgid_t kgid, kfsgid;
 
 	kgid = make_kgid(ns, gid);
 	if (!gid_valid(kgid))
 		return -EINVAL;
 
+	kfsgid = make_kfsgid(ns, gid);
+	if (!gid_valid(kfsgid))
+		return -EINVAL;
+
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
 	old = current_cred();
 
 	retval = -EPERM;
-	if (ns_capable(old->user_ns, CAP_SETGID))
-		new->gid = new->egid = new->sgid = new->fsgid = kgid;
-	else if (gid_eq(kgid, old->gid) || gid_eq(kgid, old->sgid))
-		new->egid = new->fsgid = kgid;
-	else
+	if (ns_capable(old->user_ns, CAP_SETGID)) {
+		new->gid = new->egid = new->sgid = new->kfsgid = kgid;
+		new->fsgid = kfsgid;
+	} else if (gid_eq(kgid, old->gid) || gid_eq(kgid, old->sgid)) {
+		new->egid = new->kfsgid = kgid;
+		new->fsgid = kfsgid;
+	} else {
 		goto error;
+	}
 
 	return commit_creds(new);
 
-- 
2.25.0