Commit message: KEYS: Fix a NULL pointer deref in the user-defined key type

Fix a NULL pointer deref in the user-defined key type whereby updating a
negative key into a fully instantiated key will cause an oops to occur
when the code attempts to free the non-existent old payload.

This results in an oops that looks something like the following:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: [<ffffffff81085fa1>] __call_rcu+0x11/0x13e
  PGD 3391d067 PUD 3894a067 PMD 0
  Oops: 0002 [#1] SMP
  CPU 1
  Pid: 4354, comm: keyctl Not tainted 3.1.0-fsdevel+ #1140                  /DG965RY
  RIP: 0010:[<ffffffff81085fa1>]  [<ffffffff81085fa1>] __call_rcu+0x11/0x13e
  RSP: 0018:ffff88003d591df8  EFLAGS: 00010246
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000000000006e
  RDX: ffffffff8161d0c0 RSI: 0000000000000000 RDI: 0000000000000000
  RBP: ffff88003d591e18 R08: 0000000000000000 R09: ffffffff8152fa6c
  R10: 0000000000000000 R11: 0000000000000300 R12: ffff88003b8f9538
  R13: ffffffff8161d0c0 R14: ffff88003b8f9d50 R15: ffff88003c69f908
  FS:  00007f97eb18c720(0000) GS:ffff88003bd00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000008 CR3: 000000003d47a000 CR4: 00000000000006e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
  Process keyctl (pid: 4354, threadinfo ffff88003d590000, task ffff88003c78a040)
  Stack:
   ffff88003e0ffde0 ffff88003b8f9538 0000000000000001 ffff88003b8f9d50
   ffff88003d591e28 ffffffff810860f0 ffff88003d591e68 ffffffff8117bfea
   ffff88003d591e68 ffffffff00000000 ffff88003e0ffde1 ffff88003e0ffde0
  Call Trace:
   [<ffffffff810860f0>] call_rcu_sched+0x10/0x12
   [<ffffffff8117bfea>] user_update+0x8d/0xa2
   [<ffffffff8117723a>] key_create_or_update+0x236/0x270
   [<ffffffff811789b1>] sys_add_key+0x123/0x17e
   [<ffffffff813b84bb>] system_call_fastpath+0x16/0x1b

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Neil Horman <nhorman@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2011-4110
CWE: CWE-264
Vulnerability description: The user_update function in security/keys/user_defined.c in the Linux kernel 2.6 allows local users to cause a denial of service (NULL pointer dereference and kernel oops) via vectors related to a user-defined key and "updating a negative key into a fully instantiated key."

Project: linux-2.6

int user_update(struct key *key, const void *data, size_t datalen)
{
	struct user_key_payload *upayload, *zap;
	int ret;

	ret = -EINVAL;
	if (datalen <= 0 || datalen > 32767 || !data)
		goto error;

	/* construct a replacement payload */
	ret = -ENOMEM;
	upayload = kmalloc(sizeof(*upayload) + datalen, GFP_KERNEL);
	if (!upayload)
		goto error;

	upayload->datalen = datalen;
	memcpy(upayload->data, data, datalen);

	/* check the quota and attach the new data */
	zap = upayload;

	ret = key_payload_reserve(key, datalen);

	if (ret == 0) {
		/* attach the new data, displacing the old */
		zap = key->payload.data;
		rcu_assign_pointer(key->payload.data, upayload);
		key->expiry = 0;
	}

	kfree_rcu(zap, rcu);

error:
	return ret;
}