crypto: deflate - fix spurious -ENOSPC

The code in deflate_decompress_one may erroneously return -ENOSPC even if
it didn't run out of output space. The error happens under this
condition:

- Suppose that there are two input pages, the compressed data fits into
  the first page and the zlib checksum is placed in the second page.

- The code iterates over the first page, decompresses the data and fully
  fills the destination buffer, zlib_inflate returns Z_OK becuse zlib
  hasn't seen the checksum yet.

- The outer do-while loop is iterated again, acomp_walk_next_src sets the
  input parameters to the second page containing the checksum.

- We go into the inner do-while loop, execute "dcur =
  acomp_walk_next_dst(&walk);". "dcur" is zero, so we break out of the
  loop and return -ENOSPC, despite the fact that the decompressed data
  fit into the destination buffer.

In order to fix this bug, this commit changes the logic when to report
the -ENOSPC error. We report the error if the destination buffer is empty
*and* if zlib_inflate didn't make any progress consuming the input
buffer. If zlib_inflate consumes the trailing checksum, we see that it
made progress and we will not return -ENOSPC.

Fixes: 08cabc7d3c ("crypto: deflate - Convert to acomp")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/deflate.c

@@ -164,18 +164,21 @@ static int deflate_decompress_one(struct acomp_req *req,
 
 		do {
 			unsigned int dcur;
+			unsigned long avail_in;
 
 			dcur = acomp_walk_next_dst(&walk);
-			if (!dcur) {
-				out_of_space = true;
-				break;
-			}
 
 			stream->avail_out = dcur;
 			stream->next_out = walk.dst.virt.addr;
+			avail_in = stream->avail_in;
 
 			ret = zlib_inflate(stream, Z_NO_FLUSH);
 
+			if (!dcur && avail_in == stream->avail_in) {
+				out_of_space = true;
+				break;
+			}
+
 			dcur -= stream->avail_out;
 			acomp_walk_done_dst(&walk, dcur);
 		} while (ret == Z_OK && stream->avail_in);