Heap-buffer-overflow in zend_alloc.c when assigning string with UTF-8 bytes

[bendrissou]

Description

Attached is php code:

input.php

<?php
$sx1 = new SimpleXMLElement("<root />");
$sx1->node[0] = 'node1';
$node = $sx1->node[0];

$node[0] = 'ÂÂc'; // contains repeated byte \xC2

print $sx1->asXML();
?>

Resulted in this output:

=================================================================
==3614420==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000000000 at pc 0x560456f4c11d bp 0x7ffff6e00130 sp 0x7ffff6e00120
READ of size 8 at 0x603000000000 thread T0
    #0 0x560456f4c11c in zend_mm_free_heap /home/benchmarks/php/program/Zend/zend_alloc.c:1540
    #1 0x560456f4c11c in _efree /home/benchmarks/php/program/Zend/zend_alloc.c:2773
    #2 0x5604570ce77c in zval_ptr_dtor_nogc /home/benchmarks/php/program/Zend/zend_variables.h:36
    #3 0x5604570ce77c in ZEND_ECHO_SPEC_TMPVAR_HANDLER /home/benchmarks/php/program/Zend/zend_vm_execute.h:15166
    #4 0x5604573585fa in execute_ex /home/benchmarks/php/program/Zend/zend_vm_execute.h:60745
    #5 0x5604573bd364 in zend_execute /home/benchmarks/php/program/Zend/zend_vm_execute.h:64334
    #6 0x56045757fd0f in zend_execute_script /home/benchmarks/php/program/Zend/zend.c:1943
    #7 0x560456d2423f in php_execute_script_ex /home/benchmarks/php/program/main/main.c:2594
    #8 0x560457585df8 in do_cli /home/benchmarks/php/program/sapi/cli/php_cli.c:952
    #9 0x5604561264d0 in main /home/benchmarks/php/program/sapi/cli/php_cli.c:1363
    #10 0x7f2eb21e2082 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    #11 0x560456126e7d in _start (/home/benchmarks/php/program/sapi/cli/php+0x2123e7d)

0x603000000000 is located 16 bytes to the left of 22-byte region [0x603000000010,0x603000000026)
allocated by thread T0 here:
    #0 0x7f2eb320e3ed in __interceptor_strdup ../../../../src/libsanitizer/asan/asan_interceptors.cc:445
    #1 0x5604575a6751 in save_ps_args /home/benchmarks/php/program/sapi/cli/ps_title.c:200
    #2 0x560456125d60 in main /home/benchmarks/php/program/sapi/cli/php_cli.c:1217
    #3 0x7f2eb21e2082 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x24082)

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/benchmarks/php/program/Zend/zend_alloc.c:1540 in zend_mm_free_heap
Shadow bytes around the buggy address:
  0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000:[fa]fa 00 00 06 fa fa fa 00 00 00 00 fa fa 00 00
  0x0c067fff8010: 00 04 fa fa 00 00 00 fa fa fa 00 00 00 00 fa fa
  0x0c067fff8020: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
  0x0c067fff8030: fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00
  0x0c067fff8040: 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa
  0x0c067fff8050: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==3614420==ABORTING

But I expected this output instead:

<?xml version="1.0" encoding="ISO-8859-1"?>
<root><node>??c</node></root>

To reproduce:

./php-src/sapi/cli/php input.php

Commit:

2d6b869

PHP Version

PHP 8.5.0-dev (cli) (built: May 19 2025 07:57:48) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.0-dev, Copyright (c) Zend Technologies

Operating System

Ubuntu 20.04

[ndossche]

I can't reproduce this. Can you please share your ./configure command line options and also share what version of libxml you're using? Thanks.

[bendrissou]

./configure --enable-address-sanitizer --enable-undefined-sanitizer

libxml Version => 2.9.10
libxml2 Version => 2.9.10

[bendrissou]

PHP input file is attached.

[ndossche]

I've checked out libxml2 2.9.10 and ran the input again, but still nothing. I also tied with Valgrind, but got nothing.

Looking at your ASAN report, I am surprised at the fault location however.

It fails here:

php-src/Zend/zend_alloc.c

Line 1540 in 146157d

ZEND_MM_CHECK(chunk->heap == heap, "zend_mm_heap corrupted");

Which means that chunk is bogus, but that pointer is obtained by removing the lower bits of the address (which works because of the alignment). That would mean that ptr is bogus, which is surprising.

Are you running a modified version of PHP or are you running in a special environment?
Are you able to re-run your input with the environment variable USE_ZEND_ALLOC=0 ?

[bendrissou]

I have reproduced the crash with a new system install and PHP build.

When building ./configure with no options, I get segfault.

Input in base64 code

PD9waHAKJHN4MSA9IG5ldyBTaW1wbGVYTUxFbGVtZW50KCI8cm9vdCAvPiIpOwokc3gxLT5ub2Rl
WzBdID0gJ25vZGUxJzsKJG5vZGUgPSAkc3gxLT5ub2RlWzBdOwoKJG5vZGVbMF0gPSAn8/NjJzsK
CnByaW50ICRzeDEtPmFzWE1MKCk7Cj8+Cg==

Store base64 code in file input-enc.txt, then decode:

base64 -d input-enc.txt > input.php

Next, execute program:

root@c36ec8660ad1:/usr/src/php-src# sapi/cli/php input.php
Segmentation fault (core dumped)
root@c36ec8660ad1:/usr/src/php-src# echo $?
139

I am running on Ubuntu in a docker container.

System and build info:

# sapi/cli/php -v
PHP 8.5.0-dev (cli) (built: May 19 2025 21:44:48) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.0-dev, Copyright (c) Zend Technologies

# cat /etc/os-release
PRETTY_NAME="Ubuntu 24.04.2 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04.2 LTS (Noble Numbat)"

# uname -a
Linux c36ec8660ad1 6.8.0-1028-aws #30-Ubuntu SMP Tue Apr 15 19:25:04 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

# sapi/cli/php -i | grep configure
Configure Command =>  './configure'
# sapi/cli/php -i | grep libxml   
libxml Version => 2.9.14
libxml
libxml2 Version => 2.9.14

# git rev-parse HEAD
63657df6e1301d759837744e7d3207276789a194

I have built a new clean docker image and I can confirm the crash exists. You can recreate the image by running the following dockerfile:

FROM ubuntu:24.04

ENV DEBIAN_FRONTEND=noninteractive

# Install only the essentials to build PHP
RUN apt update && apt install -y \
  git \
  autoconf \
  bison \
  re2c \
  build-essential \
  libxml2-dev \
  && apt clean

# Clone PHP source
WORKDIR /usr/src
RUN git clone https://github.com/php/php-src.git

# Build PHP with the absolute minimum
RUN apt install -y pkg-config

WORKDIR /usr/src/php-src
RUN ./buildconf

RUN apt install -y libsqlite3-dev

WORKDIR /usr/src/php-src
RUN ./configure

WORKDIR /usr/src/php-src
RUN make -j8

# Add built PHP to PATH
ENV PATH="/usr/local/bin:$PATH"

# Default command
CMD ["php", "-v"]