Fix GH-20584: Information Leak of Memory

The string added had uninitialized memory due to
php_read_stream_all_chunks() not moving the buffer position, resulting
in the same data always being overwritten instead of new data being
added to the end of the buffer.

This is backport as there is a security impact as described in
GHSA-3237-qqm7-mfv7 .

Author: ndossche

ext/standard/image.c

@@ -434,6 +434,7 @@ static size_t php_read_stream_all_chunks(php_stream *stream, char *buffer, size_
 		if (read_now < stream->chunk_size && read_total != length) {
 			return 0;
 		}
+		buffer += read_now;
 	} while (read_total < length);
 
 	return read_total;

ext/standard/tests/image/gh20584.phpt

@@ -0,0 +1,39 @@
+--TEST--
+GH-20584 (Information Leak of Memory)
+--CREDITS--
+Nikita Sveshnikov (Positive Technologies)
+--FILE--
+<?php
+// Minimal PoC: corruption/uninitialized memory leak when reading APP1 via php://filter
+$file = __DIR__ . '/gh20584.jpg';
+
+// Make APP1 large enough so it is read in multiple chunks
+$chunk = 8192;
+$tail = 123;
+$payload = str_repeat('A', $chunk) . str_repeat('B', $chunk) . str_repeat('Z',
+$tail);
+$app1Len = 2 + strlen($payload);
+
+// Minimal JPEG: SOI + APP1 + SOF0(1x1) + EOI
+$sof = "\xFF\xC0" . pack('n', 11) . "\x08" . pack('n',1) . pack('n',1) .
+"\x01\x11\x00";
+$jpeg = "\xFF\xD8" . "\xFF\xE1" . pack('n', $app1Len) . $payload . $sof .
+"\xFF\xD9";
+file_put_contents($file, $jpeg);
+
+// Read through a filter to enforce multiple reads
+$src = 'php://filter/read=string.rot13|string.rot13/resource=' . $file;
+$info = null;
+@getimagesize($src, $info);
+$exp = $payload;
+$ret = $info['APP1'];
+
+var_dump($ret === $exp);
+
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/gh20584.jpg');
+?>
+--EXPECT--
+bool(true)