[PATCH v3 17/25] commoncap: cap_bprm_set_creds(): handle fsid mappings

[Christian Brauner]

During exec the kfsids are currently reset to the effective kids. To retain the
same semantics with the introduction of fsid mappings, we lookup the userspace
effective id in the id mappings and translate the effective id into the
corresponding kfsid in the fsid mapping. This means, the behavior is unchanged
when no fsid mappings are setup and the semantics stay the same even when fsid
mappings are setup.

Cc: Jann Horn <jannh at google.com>
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
/* v2 */
- Christian Brauner <christian.brauner at ubuntu.com>:
  - Reset kfsids used for userns visible filesystems such as proc too.

/* v3 */
unchanged
---
 security/commoncap.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index f4ee0ae106b2..55e6cc24f887 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -24,6 +24,7 @@
 #include <linux/user_namespace.h>
 #include <linux/binfmts.h>
 #include <linux/personality.h>
+#include <linux/fsuidgid.h>
 
 /*
  * If a non-root user executes a setuid-root binary in
@@ -810,7 +811,10 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 	struct cred *new = bprm->cred;
 	bool effective = false, has_fcap = false, is_setid;
 	int ret;
-	kuid_t root_uid;
+	kuid_t root_uid, kfsuid;
+	kgid_t kfsgid;
+	uid_t fsuid;
+	gid_t fsgid;
 
 	if (WARN_ON(!cap_ambient_invariant_ok(old)))
 		return -EPERM;
@@ -847,8 +851,15 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 						   old->cap_permitted);
 	}
 
-	new->suid = new->fsuid = new->euid;
-	new->sgid = new->fsgid = new->egid;
+	fsuid = from_kuid_munged(new->user_ns, new->euid);
+	kfsuid = make_kfsuid(new->user_ns, fsuid);
+	new->suid = new->kfsuid = new->euid;
+	new->fsuid = kfsuid;
+
+	fsgid = from_kgid_munged(new->user_ns, new->egid);
+	kfsgid = make_kfsgid(new->user_ns, fsgid);
+	new->sgid = new->kfsgid = new->egid;
+	new->fsgid = kfsgid;
 
 	/* File caps or setid cancels ambient. */
 	if (has_fcap || is_setid)
-- 
2.25.0