Patch "KEYS: Fix race between read and revoke" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

KEYS: Fix race between read and revoke

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
keys-fix-race-between-read-and-revoke.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Howells <[email protected]>
Date: Fri, 18 Dec 2015 01:34:26 +0000
Subject: KEYS: Fix race between read and revoke

From: David Howells <[email protected]>

commit b4a1b4f5047e4f54e194681125c74c0aa64d637d upstream.

This fixes CVE-2015-7550.

There's a race between keyctl_read() and keyctl_revoke(). If the revoke happens between keyctl_read() checking the validity of a key and the key's semaphore being taken, then the key type read method will see a revoked key.

This causes a problem for the user-defined key type because it assumes in its read method that there will always be a payload in a non-revoked key and doesn't check for a NULL pointer.

Fix this by making keyctl_read() check the validity of a key after taking semaphore instead of before.

I think the bug was introduced with the original keyrings code.

This was discovered by a multithreaded test program generated by syzkaller
(http://github.com/google/syzkaller). Here's a cleaned up version:

	#include <sys/types.h>
	#include <keyutils.h>
	#include <pthread.h>
	void *thr0(void *arg)
	{
		key_serial_t key = (unsigned long)arg;
		keyctl_revoke(key);
		return 0;
	}
	void *thr1(void *arg)
	{
		key_serial_t key = (unsigned long)arg;
		char buffer[16];
		keyctl_read(key, buffer, 16);
		return 0;
	}
	int main()
	{
		key_serial_t key = add_key("user", "%", "foo", 3, KEY_SPEC_USER_KEYRING);
		pthread_t th[5];
		pthread_create(&th[0], 0, thr0, (void *)(unsigned long)key);
		pthread_create(&th[1], 0, thr1, (void *)(unsigned long)key);
		pthread_create(&th[2], 0, thr0, (void *)(unsigned long)key);
		pthread_create(&th[3], 0, thr1, (void *)(unsigned long)key);
		pthread_join(th[0], 0);
		pthread_join(th[1], 0);
		pthread_join(th[2], 0);
		pthread_join(th[3], 0);
		return 0;
	}

Build as:

cc -o keyctl-race keyctl-race.c -lkeyutils -lpthread

Run as:

while keyctl-race; do :; done

as it may need several iterations to crash the kernel. The crash can be summarised as:

	BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
	IP: [<ffffffff81279b08>] user_read+0x56/0xa3
	...
	Call Trace:
	 [<ffffffff81276aa9>] keyctl_read_key+0xb6/0xd7
	 [<ffffffff81277815>] SyS_keyctl+0x83/0xe0
	 [<ffffffff815dbb97>] entry_SYSCALL_64_fastpath+0x12/0x6f

---
 security/keys/keyctl.c |   18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

@@ -744,16 +744,16 @@ long keyctl_read_key(key_serial_t keyid,
 
 	/* the key is probably readable - now try to read it */
 can_read_key:
-	ret = key_validate(key);
-	if (ret == 0) {
-		ret = -EOPNOTSUPP;
-		if (key->type->read) {
-			/* read the data with the semaphore held (since we
-			 * might sleep) */
-			down_read(&key->sem);
+	ret = -EOPNOTSUPP;
+	if (key->type->read) {
+		/* Read the data with the semaphore held (since we might sleep)
+		 * to protect against the key being updated or revoked.
+		 */
+		down_read(&key->sem);
+		ret = key_validate(key);
+		if (ret == 0)
 			ret = key->type->read(key, buffer, buflen);
-			up_read(&key->sem);
-		}
+		up_read(&key->sem);
 	}
 
 error2:

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/keys-fix-keyring-ref-leak-in-join_session_keyring.patch queue-3.14/keys-fix-crash-when-attempt-to-garbage-collect-an-uninstantiated-keyring.patch queue-3.14/keys-fix-race-between-read-and-revoke.patch queue-3.14/keys-fix-race-between-key-destruction-and-finding-a-keyring-by-name.patch queue-3.14/keys-refcount-bug-fix.patch queue-3.14/keys-prevent-keys-from-being-removed-from-specified-keyrings.patch