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Date: Thu, 8 Jun 2017 23:49:03 +0200
From: Pali Rohár <pali.rohar@...il.com>
To: oss-security@...ts.openwall.com
Cc: security@...iadb.org,
secalert_us@...cle.com,
security@...cona.com,
Andrea Barisani <andrea@...ersepath.com>,
Michiel Beijen <michiel.beijen@...il.com>,
Alceu Rodrigues de Freitas Junior <glasswalk3r@...oo.com.br>,
cve-assign@...re.org
Subject: MySQL - use-after-free after mysql_stmt_close()
Hello!
MySQL applications written according to Oracle's MySQL documentation &
examples for mysql_stmt_close() function call are vulnerable to use-
after-free defect.
In mysql_stmt_close() documentation [1] for return value is written:
"Zero for success. Nonzero if an error occurred." And there are defined
two errors: CR_SERVER_GONE_ERROR CR_UNKNOWN_ERROR. From other parts of
documentation can be understood that error messages for statements could
be obtained by mysql_stmt_error() function [2].
Whole example of usage is written in mysql_stmt_execute() function [3].
The relevant part for mysql_stmt_close() is at the end of example:
/* Close the statement */
if (mysql_stmt_close(stmt))
{
fprintf(stderr, " failed while closing the statement\n");
fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
exit(0);
}
And here is a problem, use-after-free defect. Current implementation of
mysql_stmt_close() function unconditionally free passed statement
structure and therefore following mysql_stmt_error() call is defective
to use-after-free.
Relevant part of implementation of mysql_stmt_close() function is:
my_bool mysql_stmt_close(MYSQL_STMT *stmt)
{
int rc=0;
...
if ((rc= stmt_command(mysql, COM_STMT_CLOSE, buff, 4, stmt)))
set_stmt_errmsg(stmt, &mysql->net);
...
my_free(stmt);
return rc;
}
As you can see it stores real error message into stmt structure, but at
the end it is freed. Which means error message is no longer available
and caller is not able to read it (even via mysql_stmt_error() call).
As such defective code is in example of the usage, probably couple of
MySQL applications written according to that defective documentations
are affected to this issue.
There is reported real bug for MySQL DBI driver that is affected by this
issue [4]. Reporter probably compiled MySQL library or driver itself
with some compiler options which could detect buffer overflows and
uncovered this issue.
In April 17 I reported this issue to oCERT team and it was forwarded to
MySQL, MariaDB and Percona security teams.
MariaDB team answered that this is problem in Oracle & MySQL and their
documentation as MariaDB do not have such vulnerable example in their
documentation.
Oracle team was unwilling to tell anything, provide any information how
to handle such issue or what to do, therefore with suggestion from oCERT
I decided to make this report public and open public discussion for
other people on oss-security list how to handle this problem.
As Oracle fully ignored this problem and have not stated if problem is
in documentation, implementation or both, I see probably 3 different
solutions:
1) Documentation with examples is correct and this is how it should be
used. What is wrong is implementation.
It would mean that function mysql_stmt_error() and mysql_stmt_errno()
needs to specially handle statement pointers which were already freed by
mysql_stmt_close(). This can be done e.g. by storing hash table of
pointers and assigning for them last received error.
Or clarifying that mysql_stmt_close() does not always free passed
memory. Because from current description in documentation it is not
fully unambiguous what happen if function fails.
In this case implementation of mysql_stmt_close(), mysql_stmt_error()
and mysql_stmt_errno() are vulnerable to use-after-free defect and needs
to be fixed. And it should be assigned CVE for MySQL for this problem.
2) Implementation is correct, documentation is wrong.
Documentation needs to be fixed to properly describe how are those
functions implemented. Important note must be that if function
mysql_stmt_close() fails it is not possible to take error code via
mysql_stmt_error() or mysql_stmt_errno(). Also examples needs to be
fixed.
And then all MySQL applications which were written according to wrong
documentation needs to be fixed and for each one needs to be assigned
CVE. Number of those applications is unknown, to get it first every
application which uses libmysqlclient.so needs to be checked and
verified. What we know now is that MySQL Perl DBI is affected.
3) Documentation is wrong, but implementation of mysql_stmt_close() is
not-so-correct.
Which would mean that return value of mysql_stmt_close() is fully
meaningless as there is no way to recover from bad state. Currently
mysql_stmt_close() unconditionally free memory for statement, so no
recover is possible.
There are two options what can be done:
* Always return value zero which means no error occurred. This basically
mitigate use-after-free vulnerability in Oracle's documentation and also
all applications which were written according to documentation.
* When error occurred, do not free memory of passed structure. This
would mean that following mysql_stmt_error() call would not be affected
by use-after-free anymore.
As Oracle ignored this security related problem (***) I would like to
ask, how to handle this problem? And to which software needs to be
requested for CVE? To MySQL itself (as described in option 1)? Or to
every one software which uses MySQL (as described in option 2)?
I think you understand me, that MySQL DBD driver needs to be fixed,
ideally ASAP. Bug report on github is from April 13 [4]. And as Oracle
is not willing to do anything, I hope that people on public oss-security
list give some advice how to handle this situation.
I'm CCing all relevant security teams, when replaying please do not
forget to include them + me. Thanks!
--
[1] - https://dev.mysql.com/doc/refman/5.7/en/mysql-stmt-close.html
[2] - https://dev.mysql.com/doc/refman/5.7/en/mysql-stmt-error.html
[3] - https://dev.mysql.com/doc/refman/5.7/en/mysql-stmt-execute.html
[4] - https://github.com/perl5-dbi/DBD-mysql/issues/120
(***) - This is not a first time! Previous two security issues reported
by me were ignored too. Oracle is the worst company in handling security
issues. It is useless to report them anything. They just start threaten
if you make information about issue public. And they are not competent
to start working on it or fix it in less then 6 months! Really I suggest
to not report any security bug to Oracle, it is just wasting of time.
--
Pali Rohár
pali.rohar@...il.com
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