[PATCH v2 11/28] sys:__sys_setreuid(): handle fsid mappings

[Christian Brauner]

Switch setreuid() 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.

During setreuid() the kfsuid is set to the keuid corresponding the euid that is
requested by userspace. If the requested euid is -1 the kfsuid is reset to the
current keuid. For the latter case this means we need to lookup the
corresponding userspace euid corresponding to the current keuid in the id
mappings and translate this euid into the corresponding kfsuid in the fsid
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 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index aa379fb5e93b..4697e010bbd7 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -504,7 +504,7 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
 	const struct cred *old;
 	struct cred *new;
 	int retval;
-	kuid_t kruid, keuid;
+	kuid_t kruid, keuid, kfsuid;
 
 	kruid = make_kuid(ns, ruid);
 	keuid = make_kuid(ns, euid);
@@ -535,6 +535,13 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
 		    !uid_eq(old->suid, keuid) &&
 		    !ns_capable_setid(old->user_ns, CAP_SETUID))
 			goto error;
+		kfsuid = make_kfsuid(new->user_ns, euid);
+	} else {
+		kfsuid = kuid_to_kfsuid(new->user_ns, new->euid);
+	}
+	if (!uid_valid(kfsuid)) {
+		retval = -EINVAL;
+		goto error;
 	}
 
 	if (!uid_eq(new->uid, old->uid)) {
@@ -545,7 +552,8 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
 	if (ruid != (uid_t) -1 ||
 	    (euid != (uid_t) -1 && !uid_eq(keuid, old->uid)))
 		new->suid = new->euid;
-	new->fsuid = new->euid;
+	new->kfsuid = new->euid;
+	new->fsuid = kfsuid;
 
 	retval = security_task_fix_setuid(new, old, LSM_SETID_RE);
 	if (retval < 0)
-- 
2.25.0