[PATCH 3.16.y-ckt 128/128] KEYS: Fix keyring ref leak in join_session_keyring()

[Luis Henriques]

3.16.7-ckt23 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Yevgeny Pats <yevgeny@xxxxxxxxxxxxxxxxxxx>

commit 23567fd052a9abb6d67fe8e7a9ccdd9800a540f2 upstream.

This fixes CVE-2016-0728.

If a thread is asked to join as a session keyring the keyring that's already
set as its session, we leak a keyring reference.

This can be tested with the following program:

	#include <stddef.h>
	#include <stdio.h>
	#include <sys/types.h>
	#include <keyutils.h>

	int main(int argc, const char *argv[])
	{
		int i = 0;
		key_serial_t serial;

		serial = keyctl(KEYCTL_JOIN_SESSION_KEYRING,
				"leaked-keyring");
		if (serial < 0) {
			perror("keyctl");
			return -1;
		}

		if (keyctl(KEYCTL_SETPERM, serial,
			   KEY_POS_ALL | KEY_USR_ALL) < 0) {
			perror("keyctl");
			return -1;
		}

		for (i = 0; i < 100; i++) {
			serial = keyctl(KEYCTL_JOIN_SESSION_KEYRING,
					"leaked-keyring");
			if (serial < 0) {
				perror("keyctl");
				return -1;
			}
		}

		return 0;
	}

If, after the program has run, there something like the following line in
/proc/keys:

3f3d898f I--Q---   100 perm 3f3f0000     0     0 keyring   leaked-keyring: empty

with a usage count of 100 * the number of times the program has been run,
then the kernel is malfunctioning.  If leaked-keyring has zero usages or
has been garbage collected, then the problem is fixed.

Reported-by: Yevgeny Pats <yevgeny@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: Don Zickus <dzickus@xxxxxxxxxx>
Acked-by: Prarit Bhargava <prarit@xxxxxxxxxx>
Acked-by: Jarod Wilson <jarod@xxxxxxxxxx>
Signed-off-by: James Morris <james.l.morris@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 security/keys/process_keys.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 0cf8a130a267..4e56371f239f 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -793,6 +793,7 @@ long join_session_keyring(const char *name)
 		ret = PTR_ERR(keyring);
 		goto error2;
 	} else if (keyring == new->session_keyring) {
+		key_put(keyring);
 		ret = 0;
 		goto error2;
 	}