linux-kernel - [PATCH 3.16 170/202] KEYS: restrict /proc/keys by credentials at open time

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Message-ID: <lsq.1556377989.549443819@decadent.org.uk>
Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Eric Biggers" <ebiggers@...gle.com>,
        "David Howells" <dhowells@...hat.com>
Subject: [PATCH 3.16 170/202] KEYS: restrict /proc/keys by credentials at
 open time

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

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

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

commit 4aa68e07d845562561f5e73c04aa521376e95252 upstream.

When checking for permission to view keys whilst reading from
/proc/keys, we should use the credentials with which the /proc/keys file
was opened.  This is because, in a classic type of exploit, it can be
possible to bypass checks for the *current* credentials by passing the
file descriptor to a suid program.

Following commit 34dbbcdbf633 ("Make file credentials available to the
seqfile interfaces") we can finally fix it.  So let's do it.

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>
---
 security/keys/proc.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -193,7 +193,7 @@ static int proc_keys_show(struct seq_fil
 	struct keyring_search_context ctx = {
 		.index_key.type		= key->type,
 		.index_key.description	= key->description,
-		.cred			= current_cred(),
+		.cred			= m->file->f_cred,
 		.match			= lookup_user_key_possessed,
 		.match_data		= key,
 		.flags			= (KEYRING_SEARCH_NO_STATE_CHECK |
@@ -213,11 +213,7 @@ static int proc_keys_show(struct seq_fil
 		}
 	}
 
-	/* check whether the current task is allowed to view the key (assuming
-	 * non-possession)
-	 * - the caller holds a spinlock, and thus the RCU read lock, making our
-	 *   access to __current_cred() safe
-	 */
+	/* check whether the current task is allowed to view the key */
 	rc = key_task_permission(key_ref, ctx.cred, KEY_NEED_VIEW);
 	if (rc < 0)
 		return 0;