lib: decompress_unzstd: Limit output size

The zstd decompression code, as it is right now, will most likely fail
on 32-bit systems, as the default output buffer size causes the buffer's
end address to overflow.

Address this issue by setting a sane default to the default output size,
with a value that won't overflow the buffer's end address.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

lib/decompress_unzstd.c

@@ -178,8 +178,13 @@ static int INIT __unzstd(unsigned char *in_buf, long in_len,
 	int err;
 	size_t ret;
 
+	/*
+	 * ZSTD decompression code won't be happy if the buffer size is so big
+	 * that its end address overflows. When the size is not provided, make
+	 * it as big as possible without having the end address overflow.
+	 */
 	if (out_len == 0)
-		out_len = LONG_MAX; /* no limit */
+		out_len = UINTPTR_MAX - (uintptr_t)out_buf;
 
 	if (fill == NULL && flush == NULL)
 		/*