[Flavien GUEDEZ <flav.pg@oopacity.net>]

Hi,

After some investigations about very corrupted toast data in one 
postgres instance, I found that the pglz_decompress function (in 
common/pg_lzcompress.c) does not check correctly where it copies data 
from using memcpy(), which could result in segfault.
In this function, there are other checks to ensure that we do not copy 
after the destination end, but not if we copy data from "before the 
beginning".

Apologize, I am not a C developer and I am not used to submitting patches.
Though I have tried and attached kind of PoC with a relatively random 
corrupted payload (it was beginning with those bytes in my storage for 
obscure reasons).
Also attached a simple patch of what could be done just before the 
memcpy calls.

Regards,

Flavien

Attachments:

  [text/x-patch] check_for_corrupted.patch (822B, ../01fc0593-e31e-463d-902c-dd43174acee2@oopacity.net/2-check_for_corrupted.patch)
  download | inline diff:
--- a/src/common/pg_lzcompress.c	2023-10-18 12:34:30.616000000 +0000
+++ b/src/common/pg_lzcompress.c	2023-10-18 12:48:09.968000000 +0000
@@ -761,7 +761,12 @@
 					/*
 					 * We can safely copy "off" bytes since that clearly
 					 * results in non-overlapping source and destination.
+					 * Check for corrupt data: if we fall before the beginning of the
+					 * destination, we have problems.
 					 */
+					if (dp - off < dest) {
+						return -1;
+					}
 					memcpy(dp, dp - off, off);
 					len -= off;
 					dp += off;
@@ -792,6 +797,13 @@
 					 */
 					off += off;
 				}
+				/*
+				 * Check for corrupt data: if we fall before the beginning of the
+				 * destination, we have problems.
+				 */
+				if (dp - off < dest) {
+					return -1;
+				}
 				memcpy(dp, dp - off, len);
 				dp += len;
 			}



  [text/x-csrc] poc.c (671B, ../01fc0593-e31e-463d-902c-dd43174acee2@oopacity.net/3-poc.c)
  download | inline:
#include <ctype.h>
#include <stdio.h>

#include "postgres.h"

int
main(int argv, char **argc)
{
	char *corrupted_compressed_payload = "\x21\x1C\x44\x36\x38\x56\x40\x37\0";
	int corrupted_compressed_payload_size = strlen(corrupted_compressed_payload);
	int decompress_tmp_buff_size = 1024; // very arbitrary

	char *decompress_tmp_buff = malloc(decompress_tmp_buff_size*sizeof(char));

	int decompress_ret;
	decompress_ret = pglz_decompress(corrupted_compressed_payload, corrupted_compressed_payload_size,
		decompress_tmp_buff, decompress_tmp_buff_size, true);

	if(decompress_ret == -1) {
		printf("data is corrupted\n");
	}

	free(decompress_tmp_buff);
	
	return 0;
}