crypto: xts,lrw - fix out-of-bounds write after kmalloc failure

In the generic XTS and LRW algorithms, for input data > 128 bytes, a
temporary buffer is allocated to hold the values to be XOR'ed with the
data before and after encryption or decryption.  If the allocation
fails, the fixed-size buffer embedded in the request buffer is meant to
be used as a fallback --- resulting in more calls to the ECB algorithm,
but still producing the correct result.  However, we weren't correctly
limiting subreq->cryptlen in this case, resulting in pre_crypt()
overrunning the embedded buffer.  Fix this by setting subreq->cryptlen
correctly.

Fixes: f1c131b4 ("crypto: xts - Convert to skcipher")
Fixes: 700cb3f5 ("crypto: lrw - Convert to skcipher")
Cc: stable@vger.kernel.org # v4.10+
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/lrw.c

@@ -286,8 +286,11 @@ static int init_crypt(struct skcipher_request *req, crypto_completion_t done)
 
 	subreq->cryptlen = LRW_BUFFER_SIZE;
 	if (req->cryptlen > LRW_BUFFER_SIZE) {
-		subreq->cryptlen = min(req->cryptlen, (unsigned)PAGE_SIZE);
-		rctx->ext = kmalloc(subreq->cryptlen, gfp);
+		unsigned int n = min(req->cryptlen, (unsigned int)PAGE_SIZE);
+
+		rctx->ext = kmalloc(n, gfp);
+		if (rctx->ext)
+			subreq->cryptlen = n;
 	}
 
 	rctx->src = req->src;

crypto/xts.c

@@ -230,8 +230,11 @@ static int init_crypt(struct skcipher_request *req, crypto_completion_t done)
 
 	subreq->cryptlen = XTS_BUFFER_SIZE;
 	if (req->cryptlen > XTS_BUFFER_SIZE) {
-		subreq->cryptlen = min(req->cryptlen, (unsigned)PAGE_SIZE);
-		rctx->ext = kmalloc(subreq->cryptlen, gfp);
+		unsigned int n = min(req->cryptlen, (unsigned int)PAGE_SIZE);
+
+		rctx->ext = kmalloc(n, gfp);
+		if (rctx->ext)
+			subreq->cryptlen = n;
 	}
 
 	rctx->src = req->src;