crypto: ablkcipher - fix crash flushing dcache in error path

Like the skcipher_walk and blkcipher_walk cases:

scatterwalk_done() is only meant to be called after a nonzero number of
bytes have been processed, since scatterwalk_pagedone() will flush the
dcache of the *previous* page.  But in the error case of
ablkcipher_walk_done(), e.g. if the input wasn't an integer number of
blocks, scatterwalk_done() was actually called after advancing 0 bytes.
This caused a crash ("BUG: unable to handle kernel paging request")
during '!PageSlab(page)' on architectures like arm and arm64 that define
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE, provided that the input was
page-aligned as in that case walk->offset == 0.

Fix it by reorganizing ablkcipher_walk_done() to skip the
scatterwalk_advance() and scatterwalk_done() if an error has occurred.

Reported-by: Liu Chao <liuchao741@huawei.com>
Fixes: bf06099db18a ("crypto: skcipher - Add ablkcipher_walk interfaces")
Cc: <stable@vger.kernel.org> # v2.6.35+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ablkcipher.c

@@ -71,11 +71,9 @@ static inline u8 *ablkcipher_get_spot(u8 *start, unsigned int len)
 	return max(start, end_page);
 }
 
-static inline unsigned int ablkcipher_done_slow(struct ablkcipher_walk *walk,
-						unsigned int bsize)
+static inline void ablkcipher_done_slow(struct ablkcipher_walk *walk,
+					unsigned int n)
 {
-	unsigned int n = bsize;
-
 	for (;;) {
 		unsigned int len_this_page = scatterwalk_pagelen(&walk->out);
 
@@ -87,17 +85,13 @@ static inline unsigned int ablkcipher_done_slow(struct ablkcipher_walk *walk,
 		n -= len_this_page;
 		scatterwalk_start(&walk->out, sg_next(walk->out.sg));
 	}
-
-	return bsize;
 }
 
-static inline unsigned int ablkcipher_done_fast(struct ablkcipher_walk *walk,
-						unsigned int n)
+static inline void ablkcipher_done_fast(struct ablkcipher_walk *walk,
+					unsigned int n)
 {
 	scatterwalk_advance(&walk->in, n);
 	scatterwalk_advance(&walk->out, n);
-
-	return n;
 }
 
 static int ablkcipher_walk_next(struct ablkcipher_request *req,
@@ -107,39 +101,40 @@ int ablkcipher_walk_done(struct ablkcipher_request *req,
 			 struct ablkcipher_walk *walk, int err)
 {
 	struct crypto_tfm *tfm = req->base.tfm;
-	unsigned int nbytes = 0;
+	unsigned int n; /* bytes processed */
+	bool more;
 
-	if (likely(err >= 0)) {
-		unsigned int n = walk->nbytes - err;
+	if (unlikely(err < 0))
+		goto finish;
 
-		if (likely(!(walk->flags & ABLKCIPHER_WALK_SLOW)))
-			n = ablkcipher_done_fast(walk, n);
-		else if (WARN_ON(err)) {
-			err = -EINVAL;
-			goto err;
-		} else
-			n = ablkcipher_done_slow(walk, n);
+	n = walk->nbytes - err;
+	walk->total -= n;
+	more = (walk->total != 0);
 
-		nbytes = walk->total - n;
-		err = 0;
+	if (likely(!(walk->flags & ABLKCIPHER_WALK_SLOW))) {
+		ablkcipher_done_fast(walk, n);
+	} else {
+		if (WARN_ON(err)) {
+			/* unexpected case; didn't process all bytes */
+			err = -EINVAL;
+			goto finish;
+		}
+		ablkcipher_done_slow(walk, n);
 	}
 
-	scatterwalk_done(&walk->in, 0, nbytes);
-	scatterwalk_done(&walk->out, 1, nbytes);
-
-err:
-	walk->total = nbytes;
-	walk->nbytes = nbytes;
+	scatterwalk_done(&walk->in, 0, more);
+	scatterwalk_done(&walk->out, 1, more);
 
-	if (nbytes) {
+	if (more) {
 		crypto_yield(req->base.flags);
 		return ablkcipher_walk_next(req, walk);
 	}
-
+	err = 0;
+finish:
+	walk->nbytes = 0;
 	if (walk->iv != req->info)
 		memcpy(req->info, walk->iv, tfm->crt_ablkcipher.ivsize);
 	kfree(walk->iv_buffer);
-
 	return err;
 }
 EXPORT_SYMBOL_GPL(ablkcipher_walk_done);