[Bug 1939853] [NEW] mysqli: Using a cursor with get_result() and prepared statements causes a segmentation fault

Thu Jan 13 00:21:38 UTC 2022

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[Impact]

If you attempt to use a prepared statement with the mysqli database
driver to create a cursor which you then execute() and fetch rows with
get_result() and then fetch_assoc(), php with hit a segmentation fault
on every query and terminate.

This is because cursors aren't actually implemented for prepared
statements for the mysqli driver in php 7.2 and 7.4, the versions in
Bionic and Focal. When we try and use a cursor, we just segfault on a
type mismatch when the cursor calls fetch_row().

The fix comes in two forms. The first commit fixes the segfault and
makes php return an error to the user, and the second commit implements
support for cursors on prepared statements. When combined, these commits
fix the issue.

A workaround is to not use prepared statements, and instead use query()
directly.

[Test case]

Install PHP and mysql-client:

Focal:
$ sudo apt install php7.4 php7.4-mysql mysql-client

Bionic:
$ sudo apt install php7.2 php7.2-mysql mysql-client

Next, install and configure mysql 5.3:

$ sudo apt install libncurses5 libaio1 libmecab2
$ wget https://dev.mysql.com/get/Downloads/MySQL-5.7/mysql-5.7.35-linux-glibc2.12-x86_64.tar.gz

$ groupadd mysql
$ useradd -r -g mysql -s /bin/false mysql
$ cd /usr/local
$ tar zxvf /home/ubuntu/mysql-5.7.35-linux-glibc2.12-x86_64.tar.gz
$ ln -s mysql-5.7.35-linux-glibc2.12-x86_64 mysql
$ cd mysql
$ mkdir mysql-files
$ chown mysql:mysql mysql-files
$ chmod 750 mysql-files
$ bin/mysqld --initialize --user=mysql
$ bin/mysql_ssl_rsa_setup
$ bin/mysqld_safe --user=mysql &

# [Note] A temporary password is generated for root at localhost:
*rjfy#_w(8kM

Access the DBMS and add users, databases and tables, along with some
data:

$ mysql -h 127.0.0.1 -P 3306 -u root -p

ALTER USER 'root'@'localhost' IDENTIFIED BY 'ubuntu';

CREATE DATABASE ubuntu_releases;
use ubuntu_releases;
CREATE TABLE ubuntu_releases (year INT, month INT, name VARCHAR(20)) ;
INSERT INTO ubuntu_releases VALUES (21, 04, 'hirsute');
INSERT INTO ubuntu_releases VALUES (20, 10, 'groovy');
INSERT INTO ubuntu_releases VALUES (20, 04, 'focal');
CREATE USER 'ubuntu' IDENTIFIED BY 'ubuntu';
GRANT ALL PRIVILEGES ON ubuntu_releases.* TO 'ubuntu';
exit

Save the following script to testcase.php:

<?php
 $dbConn= new mysqli("127.0.0.1", "ubuntu", "ubuntu", "ubuntu_releases", "3306");
 $SQL="SELECT * from ubuntu_releases";
 $stmt=$dbConn->prepare($SQL);
 $stmt->attr_set(MYSQLI_STMT_ATTR_CURSOR_TYPE, MYSQLI_CURSOR_TYPE_READ_ONLY);
 $stmt->execute();
 if ($stmt) {
  $res = $stmt->get_result();
  while($row = $res->fetch_assoc()) {
   echo json_encode($row) . "\n";
  }
 }
 $dbConn->close()
?>

Run the php script:

$ php testcase.php
Segmentation fault (core dumped)

A ppa with test packages is available below:

https://launchpad.net/~mruffell/+archive/ubuntu/sf315485-test

When you install the test packages, you should see:

$ php testcase.php
{"year":21,"month":4,"name":"hirsute"}
{"year":20,"month":10,"name":"groovy"}
{"year":20,"month":4,"name":"focal"}

[Where problems can occur]

We are changing the behaviour of how cursors work in the mysqli database
driver backend. Luckily, we are only changing how they work for prepared
statements in mysqli, and it doesn't affect any other database driver
backend, or regular queries with mysqli.

Since attempting to use a cursor with prepared statements on mysqli
backend results in a segfault due to it not being implemented, there
won't be any users that are using cursors with prepared statements,
since their applications would crash. Adding support likely won't break
any existing users, as attempting to use such features before would
result in a hard crash, versus making existing code start working or
behave differently.

There is still risk these changes could introduce a regression, and it
would be restricted to users using the mysqli database driver, which I
imagine are a significant amount of the userbase, due to the popularity
of mysql and mariadb. If a regression were to occur, users might need to
change their database driver interface code, or in worst case, change to
direct queries from prepared statements while a fix is created.

[Other Info]

The following commit fixes the segmentation fault and changes it to an
not implemented error:

commit b5481defe64c991d0e4307372d69c0ea3cd83378
Author: Dharman <tekiela246 at gmail.com>
Date:   Thu Sep 17 12:35:26 2020 +0100
Subject: Fix bug #72413: Segfault with get_result and PS cursors
Link: https://github.com/php/php-src/commit/b5481defe64c991d0e4307372d69c0ea3cd83378

The following commit implements support for cursors on prepared
statements:

commit bc166844e37a6e1531a18dc0916fbe508152fc6c
Author: Nikita Popov <nikita.ppv at gmail.com>
Date:   Wed Dec 16 12:12:06 2020 +0100
Subject: MySQLnd: Support cursors in store/get result
Link: https://github.com/php/php-src/commit/bc166844e37a6e1531a18dc0916fbe508152fc6c

Both of these commits landed in php 7.4.13 and 7.4.15, which means they
are already present in hirsute 7.5.16 and impish 8.0.

Both commits required backports to remove the NEWS hunk, and the second
commit needed indentation adjustments. For Bionic, the second commit
required two changes. There are two hunks in ext/mysqlnd/mysqlnd_ps.c
particularly around the function
mysqlnd_stmt_send_cursor_fetch_command() and a small change to
ext/mysqli/tests/bug77935.phpt, moving ); after SQL block.

** Affects: php-luasandbox (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: php7.4 (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: php7.2 (Ubuntu Bionic)
     Importance: Medium
     Assignee: Matthew Ruffell (mruffell)
         Status: Fix Released

** Affects: php7.4 (Ubuntu Focal)
     Importance: Medium
     Assignee: Matthew Ruffell (mruffell)
         Status: Fix Released

** Tags: patch sts sts-sponsor sts-sponsor-ddstreet verification-done-bionic verification-done-focal
-- 
mysqli: Using a cursor with get_result() and prepared statements causes a segmentation fault
https://bugs.launchpad.net/bugs/1939853
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