linux-kernel - [PATCH 3.16 111/204] lib/digsig: fix dereference of NULL user_key

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Date:   Thu, 28 Dec 2017 17:05:44 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David Howells" <dhowells@...hat.com>,
        "Dmitry Kasatkin" <dmitry.kasatkin@...el.com>,
        "Eric Biggers" <ebiggers@...gle.com>,
        "James Morris" <james.l.morris@...cle.com>
Subject: [PATCH 3.16 111/204] lib/digsig: fix dereference of NULL
 user_key_payload

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

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

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

commit 192cabd6a296cbc57b3d8c05c4c89d87fc102506 upstream.

digsig_verify() requests a user key, then accesses its payload.
However, a revoked key has a NULL payload, and we failed to check for
this.  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.

Fixes: 051dbb918c7f ("crypto: digital signature verification support")
Reviewed-by: James Morris <james.l.morris@...cle.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@...el.com>
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: David Howells <dhowells@...hat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 lib/digsig.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/lib/digsig.c
+++ b/lib/digsig.c
@@ -86,6 +86,12 @@ static int digsig_verify_rsa(struct key
 	down_read(&key->sem);
 	ukp = key->payload.data;
 
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		err = -EKEYREVOKED;
+		goto err1;
+	}
+
 	if (ukp->datalen < sizeof(*pkh))
 		goto err1;