crypto: algif_skcipher - Do not dereference ctx without socket lock

commit 6454c2b8 upstream.

Any access to non-constant bits of the private context must be
done under the socket lock, in particular, this includes ctx->req.

This patch moves such accesses under the lock, and fetches the
tfm from the parent socket which is guaranteed to be constant,
rather than from ctx->req.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[bwh: Backported to 3.2:
 - Drop changes to skcipher_recvmsg_async
 - s/skcipher/ablkcipher/ in many places
 - s/skc->skcipher/skc->base/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

crypto/algif_skcipher.c

@@ -249,8 +249,11 @@ static int skcipher_sendmsg(struct kiocb *unused, struct socket *sock,
 {
 	struct sock *sk = sock->sk;
 	struct alg_sock *ask = alg_sk(sk);
+	struct sock *psk = ask->parent;
+	struct alg_sock *pask = alg_sk(psk);
 	struct skcipher_ctx *ctx = ask->private;
-	struct crypto_ablkcipher *tfm = crypto_ablkcipher_reqtfm(&ctx->req);
+	struct ablkcipher_tfm *skc = pask->private;
+	struct crypto_ablkcipher *tfm = skc->base;
 	unsigned ivsize = crypto_ablkcipher_ivsize(tfm);
 	struct skcipher_sg_list *sgl;
 	struct af_alg_control con = {};