[PATCH 3.2 23/94] KEYS: prevent creating a different user's keyrings

[Ben Hutchings]

3.2.97-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 237bbd29f7a049d310d907f4b2716a7feef9abf3 upstream.

It was possible for an unprivileged user to create the user and user
session keyrings for another user.  For example:

    sudo -u '#3000' sh -c 'keyctl add keyring _uid.4000 "" @u
                           keyctl add keyring _uid_ses.4000 "" @u
                           sleep 15' &
    sleep 1
    sudo -u '#4000' keyctl describe @u
    sudo -u '#4000' keyctl describe @us

This is problematic because these "fake" keyrings won't have the right
permissions.  In particular, the user who created them first will own
them and will have full access to them via the possessor permissions,
which can be used to compromise the security of a user's keys:

    -4: alswrv-----v------------  3000     0 keyring: _uid.4000
    -5: alswrv-----v------------  3000     0 keyring: _uid_ses.4000

Fix it by marking user and user session keyrings with a flag
KEY_FLAG_UID_KEYRING.  Then, when searching for a user or user session
keyring by name, skip all keyrings that don't have the flag set.

Fixes: 69664cf16af4 ("keys: don't generate user and user session keyrings unless they're accessed")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -155,6 +155,7 @@ struct key {
 #define KEY_FLAG_IN_QUOTA	3	/* set if key consumes quota */
 #define KEY_FLAG_USER_CONSTRUCT	4	/* set if key is being constructed in userspace */
 #define KEY_FLAG_NEGATIVE	5	/* set if key is negative */
+#define KEY_FLAG_UID_KEYRING	11	/* set if key is a user or user session keyring */
 
 	/* the description string
 	 * - this is used to match a key against search criteria
@@ -196,6 +197,7 @@ extern struct key *key_alloc(struct key_
 #define KEY_ALLOC_IN_QUOTA	0x0000	/* add to quota, reject if would overrun */
 #define KEY_ALLOC_QUOTA_OVERRUN	0x0001	/* add to quota, permit even if overrun */
 #define KEY_ALLOC_NOT_IN_QUOTA	0x0002	/* not in quota */
+#define KEY_ALLOC_UID_KEYRING	0x0010	/* allocating a user or user session keyring */
 
 extern void key_revoke(struct key *key);
 extern void key_put(struct key *key);
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -124,7 +124,7 @@ extern key_ref_t search_process_keyrings
 					 key_match_func_t match,
 					 const struct cred *cred);
 
-extern struct key *find_keyring_by_name(const char *name, bool skip_perm_check);
+extern struct key *find_keyring_by_name(const char *name, bool uid_keyring);
 
 extern int install_user_keyrings(void);
 extern int install_thread_keyring_to_cred(struct cred *);
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -305,6 +305,8 @@ struct key *key_alloc(struct key_type *t
 
 	if (!(flags & KEY_ALLOC_NOT_IN_QUOTA))
 		key->flags |= 1 << KEY_FLAG_IN_QUOTA;
+	if (flags & KEY_ALLOC_UID_KEYRING)
+		key->flags |= 1 << KEY_FLAG_UID_KEYRING;
 
 	memset(&key->type_data, 0, sizeof(key->type_data));
 
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -550,15 +550,15 @@ found:
 /*
  * Find a keyring with the specified name.
  *
- * All named keyrings in the current user namespace are searched, provided they
- * grant Search permission directly to the caller (unless this check is
- * skipped).  Keyrings whose usage points have reached zero or who have been
- * revoked are skipped.
+ * Only keyrings that have nonzero refcount, are not revoked, and are owned by a
+ * user in the current user namespace are considered.  If @uid_keyring is %true,
+ * the keyring additionally must have been allocated as a user or user session
+ * keyring; otherwise, it must grant Search permission directly to the caller.
  *
  * Returns a pointer to the keyring with the keyring's refcount having being
  * incremented on success.  -ENOKEY is returned if a key could not be found.
  */
-struct key *find_keyring_by_name(const char *name, bool skip_perm_check)
+struct key *find_keyring_by_name(const char *name, bool uid_keyring)
 {
 	struct key *keyring;
 	int bucket;
@@ -586,10 +586,15 @@ struct key *find_keyring_by_name(const c
 			if (strcmp(keyring->description, name) != 0)
 				continue;
 
-			if (!skip_perm_check &&
-			    key_permission(make_key_ref(keyring, 0),
-					   KEY_SEARCH) < 0)
-				continue;
+			if (uid_keyring) {
+				if (!test_bit(KEY_FLAG_UID_KEYRING,
+					      &keyring->flags))
+					continue;
+			} else {
+				if (key_permission(make_key_ref(keyring, 0),
+						   KEY_SEARCH) < 0)
+					continue;
+			}
 
 			/* we've got a match but we might end up racing with
 			 * key_cleanup() if the keyring is currently 'dead'
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -72,7 +72,9 @@ int install_user_keyrings(void)
 		uid_keyring = find_keyring_by_name(buf, true);
 		if (IS_ERR(uid_keyring)) {
 			uid_keyring = keyring_alloc(buf, user->uid, (gid_t) -1,
-						    cred, KEY_ALLOC_IN_QUOTA,
+						    cred,
+						    KEY_ALLOC_UID_KEYRING |
+							KEY_ALLOC_IN_QUOTA,
 						    NULL);
 			if (IS_ERR(uid_keyring)) {
 				ret = PTR_ERR(uid_keyring);
@@ -88,7 +90,10 @@ int install_user_keyrings(void)
 		if (IS_ERR(session_keyring)) {
 			session_keyring =
 				keyring_alloc(buf, user->uid, (gid_t) -1,
-					      cred, KEY_ALLOC_IN_QUOTA, NULL);
+					      cred,
+					      KEY_ALLOC_UID_KEYRING |
+						  KEY_ALLOC_IN_QUOTA,
+					      NULL);
 			if (IS_ERR(session_keyring)) {
 				ret = PTR_ERR(session_keyring);
 				goto error_release;