erofs: fix out-of-bound read for shifted uncompressed block

rq->out[1] should be valid before accessing. Otherwise,
in very rare cases, out-of-bound dirty onstack rq->out[1]
can equal to *in and lead to unintended memmove behavior.

Link: https://lore.kernel.org/r/20200107022546.19432-1-gaoxiang25@huawei.com
Fixes: 7fc45dbc ("staging: erofs: introduce generic decompression backend")
Cc: <stable@vger.kernel.org> # 5.3+
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>

fs/erofs/decompressor.c

@@ -306,24 +306,22 @@ static int z_erofs_shifted_transform(const struct z_erofs_decompress_req *rq,
 	}
 
 	src = kmap_atomic(*rq->in);
-	if (!rq->out[0]) {
-		dst = NULL;
-	} else {
+	if (rq->out[0]) {
 		dst = kmap_atomic(rq->out[0]);
 		memcpy(dst + rq->pageofs_out, src, righthalf);
+		kunmap_atomic(dst);
 	}
 
+	if (nrpages_out == 2) {
+		DBG_BUGON(!rq->out[1]);
 		if (rq->out[1] == *rq->in) {
 			memmove(src, src + righthalf, rq->pageofs_out);
-	} else if (nrpages_out == 2) {
-		if (dst)
-			kunmap_atomic(dst);
-		DBG_BUGON(!rq->out[1]);
+		} else {
 			dst = kmap_atomic(rq->out[1]);
 			memcpy(dst, src + righthalf, rq->pageofs_out);
-	}
-	if (dst)
 			kunmap_atomic(dst);
+		}
+	}
 	kunmap_atomic(src);
 	return 0;
 }