linux-kernel - [PATCH 3.2 52/94] KEYS: encrypted: fix dereference of NULL user_key

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Date:   Thu, 28 Dec 2017 16:59:12 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "James Morris" <james.l.morris@...cle.com>,
        "David Safford" <safford@...ibm.com>,
        "Mimi Zohar" <zohar@...ux.vnet.ibm.com>,
        "David Howells" <dhowells@...hat.com>,
        "Eric Biggers" <ebiggers@...gle.com>
Subject: [PATCH 3.2 52/94] KEYS: encrypted: fix dereference of NULL
 user_key_payload

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

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

From: Eric Biggers <ebiggers@...gle.com>

commit 13923d0865ca96312197962522e88bc0aedccd74 upstream.

A key of type "encrypted" references a "master key" which is used to
encrypt and decrypt the encrypted key's payload.  However, when we
accessed the master key's payload, we failed to handle the case where
the master key has been revoked, which sets the payload pointer to NULL.
Note that request_key() *does* skip revoked keys, but there is still a
window where the key can be revoked before we acquire its semaphore.

Fix it by checking for a NULL payload, treating it like a key which was
already revoked at the time it was requested.

This was an issue for master keys of type "user" only.  Master keys can
also be of type "trusted", but those cannot be revoked.

Fixes: 7e70cb497850 ("keys: add new key-type encrypted")
Reviewed-by: James Morris <james.l.morris@...cle.com>
Cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc: David Safford <safford@...ibm.com>
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: David Howells <dhowells@...hat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 security/keys/encrypted-keys/encrypted.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -315,6 +315,13 @@ static struct key *request_user_key(cons

 	down_read(&ukey->sem);
 	upayload = rcu_dereference(ukey->payload.data);
+	if (!upayload) {
+		/* key was revoked before we acquired its semaphore */
+		up_read(&ukey->sem);
+		key_put(ukey);
+		ukey = ERR_PTR(-EKEYREVOKED);
+		goto error;
+	}
 	*master_key = upayload->data;
 	*master_keylen = upayload->datalen;
 error: