crypto: skcipher - set walk.iv for zero-length inputs

All the ChaCha20 algorithms as well as the ARM bit-sliced AES-XTS
algorithms call skcipher_walk_virt(), then access the IV (walk.iv)
before checking whether any bytes need to be processed (walk.nbytes).

But if the input is empty, then skcipher_walk_virt() doesn't set the IV,
and the algorithms crash trying to use the uninitialized IV pointer.

Fix it by setting the IV earlier in skcipher_walk_virt().  Also fix it
for the AEAD walk functions.

This isn't a perfect solution because we can't actually align the IV to
->cra_alignmask unless there are bytes to process, for one because the
temporary buffer for the aligned IV is freed by skcipher_walk_done(),
which is only called when there are bytes to process.  Thus, algorithms
that require aligned IVs will still need to avoid accessing the IV when
walk.nbytes == 0.  Still, many algorithms/architectures are fine with
IVs having any alignment, and even for those that aren't, a misaligned
pointer bug is much less severe than an uninitialized pointer bug.

This change also matches the behavior of the older blkcipher_walk API.

Fixes: 0cabf2af ("crypto: skcipher - Fix crash on zero-length input")
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

crypto/skcipher.c

@@ -449,6 +449,8 @@ static int skcipher_walk_skcipher(struct skcipher_walk *walk,
 
 	walk->total = req->cryptlen;
 	walk->nbytes = 0;
+	walk->iv = req->iv;
+	walk->oiv = req->iv;
 
 	if (unlikely(!walk->total))
 		return 0;
@@ -456,9 +458,6 @@ static int skcipher_walk_skcipher(struct skcipher_walk *walk,
 	scatterwalk_start(&walk->in, req->src);
 	scatterwalk_start(&walk->out, req->dst);
 
-	walk->iv = req->iv;
-	walk->oiv = req->iv;
-
 	walk->flags &= ~SKCIPHER_WALK_SLEEP;
 	walk->flags |= req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP ?
 		       SKCIPHER_WALK_SLEEP : 0;
@@ -510,6 +509,8 @@ static int skcipher_walk_aead_common(struct skcipher_walk *walk,
 	int err;
 
 	walk->nbytes = 0;
+	walk->iv = req->iv;
+	walk->oiv = req->iv;
 
 	if (unlikely(!walk->total))
 		return 0;
@@ -525,9 +526,6 @@ static int skcipher_walk_aead_common(struct skcipher_walk *walk,
 	scatterwalk_done(&walk->in, 0, walk->total);
 	scatterwalk_done(&walk->out, 0, walk->total);
 
-	walk->iv = req->iv;
-	walk->oiv = req->iv;
-
 	if (req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP)
 		walk->flags |= SKCIPHER_WALK_SLEEP;
 	else