diff --git a/crypto/blkcipher.c b/crypto/blkcipher.c
index 7755834..97586af 100644
--- a/crypto/blkcipher.c
+++ b/crypto/blkcipher.c
@@ -59,11 +59,13 @@
 	scatterwalk_unmap(walk->dst.virt.addr, 1);
 }
 
+/* Get a spot of the specified length that does not straddle a page.
+ * The caller needs to ensure that there is enough space for this operation.
+ */
 static inline u8 *blkcipher_get_spot(u8 *start, unsigned int len)
 {
-	if (offset_in_page(start + len) < len)
-		return (u8 *)((unsigned long)(start + len) & PAGE_MASK);
-	return start;
+	u8 *end_page = (u8 *)(((unsigned long)(start + len - 1)) & PAGE_MASK);
+	return start < end_page ? start : end_page;
 }
 
 static inline unsigned int blkcipher_done_slow(struct crypto_blkcipher *tfm,
@@ -155,7 +157,8 @@
 	if (walk->buffer)
 		goto ok;
 
-	n = bsize * 2 + (alignmask & ~(crypto_tfm_ctx_alignment() - 1));
+	n = bsize * 3 - (alignmask + 1) +
+	    (alignmask & ~(crypto_tfm_ctx_alignment() - 1));
 	walk->buffer = kmalloc(n, GFP_ATOMIC);
 	if (!walk->buffer)
 		return blkcipher_walk_done(desc, walk, -ENOMEM);