erofs: avoid infinite loop due to incomplete zstd-compressed data

Currently, the decompression logic incorrectly spins if compressed
data is truncated in crafted (deliberately corrupted) images.

Fixes: 7c35de4 ("erofs: Zstandard compression support")
Reported-by: Robert Morris <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/r/50958.1761605413@localhost
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chunhai Guo <guochunhai@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>

Author: hsiangkao

fs/erofs/decompressor_zstd.c

@@ -172,7 +172,6 @@ static int z_erofs_zstd_decompress(struct z_erofs_decompress_req *rq,
 	dctx.bounce = strm->bounce;
 
 	do {
-		dctx.avail_out = out_buf.size - out_buf.pos;
 		dctx.inbuf_sz = in_buf.size;
 		dctx.inbuf_pos = in_buf.pos;
 		err = z_erofs_stream_switch_bufs(&dctx, &out_buf.dst,
@@ -188,14 +187,18 @@ static int z_erofs_zstd_decompress(struct z_erofs_decompress_req *rq,
 		in_buf.pos = dctx.inbuf_pos;
 
 		zerr = zstd_decompress_stream(stream, &out_buf, &in_buf);
-		if (zstd_is_error(zerr) || (!zerr && rq->outputsize)) {
+		dctx.avail_out = out_buf.size - out_buf.pos;
+		if (zstd_is_error(zerr) ||
+		    ((rq->outputsize + dctx.avail_out) && (!zerr || (zerr > 0 &&
+				!(rq->inputsize + in_buf.size - in_buf.pos))))) {
 			erofs_err(sb, "failed to decompress in[%u] out[%u]: %s",
 				  rq->inputsize, rq->outputsize,
-				  zerr ? zstd_get_error_name(zerr) : "unexpected end of stream");
+				  zstd_is_error(zerr) ? zstd_get_error_name(zerr) :
+					"unexpected end of stream");
 			err = -EFSCORRUPTED;
 			break;
 		}
-	} while (rq->outputsize || out_buf.pos < out_buf.size);
+	} while (rq->outputsize + dctx.avail_out);
 
 	if (dctx.kout)
 		kunmap_local(dctx.kout);