crypto: skcipher - Fix blkcipher walk OOM crash

commit acdb04d0 upstream.

When we need to allocate a temporary blkcipher_walk_next and it
fails, the code is supposed to take the slow path of processing
the data block by block.  However, due to an unrelated change
we instead end up dereferencing the NULL pointer.

This patch fixes it by moving the unrelated bsize setting out
of the way so that we enter the slow path as inteded.

Fixes: 7607bd8f ("[CRYPTO] blkcipher: Added blkcipher_walk_virt_block")
Reported-by: xiakaixu <xiakaixu@huawei.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

crypto/blkcipher.c

@@ -233,6 +233,8 @@ static int blkcipher_walk_next(struct blkcipher_desc *desc,
 		return blkcipher_walk_done(desc, walk, -EINVAL);
 	}
 
+	bsize = min(walk->walk_blocksize, n);
+
 	walk->flags &= ~(BLKCIPHER_WALK_SLOW | BLKCIPHER_WALK_COPY |
 			 BLKCIPHER_WALK_DIFF);
 	if (!scatterwalk_aligned(&walk->in, walk->alignmask) ||
@@ -245,7 +247,6 @@ static int blkcipher_walk_next(struct blkcipher_desc *desc,
 		}
 	}
 
-	bsize = min(walk->walk_blocksize, n);
 	n = scatterwalk_clamp(&walk->in, n);
 	n = scatterwalk_clamp(&walk->out, n);