oss-security - Problems in automatic crash analysis frameworks

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Date: Tue, 14 Apr 2015 06:30:41 -0700
From: Tavis Ormandy <taviso@...gle.com>
To: oss-security@...ts.openwall.com
Subject: Problems in automatic crash analysis frameworks

Hello, this is CVE-2015-1318 and CVE-2015-1862 (essentially the same
bugs in two different implementations, apport and abrt respectively).
These were discussed on the vendors list last week.

If the first character of kern.core_pattern sysctl is a pipe, the
kernel will invoke the specified program, and pass it the core on
stdin. Apport (Ubuntu) and Abrt (Fedora) use this feature to analyze
and log crashes.

Since the introduction of containers, Abrt and Apport have attempted
to transparently handle namespaces by chrooting into the same root as
the crashing program [1] [2]. Unfortunately, this is incorrect because
root cannot safely execve() after a chroot into a user specified
directory.

Furthermore, Abrt suffers from numerous race conditions and symlink
problems from trusting unprivileged programs. For example, the code
below (and lots of similar code) is vulnerable to a filesystem race
where a user unlinks the file after the copy but before the chown.

https://github.com/abrt/abrt/blob/master/src/hooks/abrt-hook-ccpp.c#L634

        strcpy(source_filename + source_base_ofs, "maps");
        strcpy(dest_base, FILENAME_MAPS);
        copy_file(source_filename, dest_filename, DEFAULT_DUMP_DIR_MODE);
        IGNORE_RESULT(chown(dest_filename, dd->dd_uid, dd->dd_gid));

This code trusts various symlinks in /tmp without validation:

https://github.com/abrt/abrt/blob/master/src/hooks/abrt-hook-ccpp.c#L806

        char *java_log = xasprintf("/tmp/jvm-%lu/hs_error.log", (long)pid);
        int src_fd = open(java_log, O_RDONLY);
        free(java_log);

This code trusts the /proc/pid/exe symlink, even though it is possible
to link it anywhere you want.

https://github.com/abrt/abrt/blob/master/src/hooks/abrt-hook-ccpp.c#L368

        sprintf(buf, "/proc/%lu/exe", (long)pid);
        int src_fd_binary = open(buf, O_RDONLY); /* might fail and
return -1, it's ok */

This code trusts the attacker controlled root symlink and copies files from it.

https://github.com/abrt/libreport/blob/master/src/lib/dump_dir.c#L671

        if (chroot_dir)
            copy_file_from_chroot(dd, FILENAME_OS_INFO_IN_ROOTDIR,
chroot_dir, "/etc/os-release");

This instructs librpm to trust an unprivileged root symlink:

https://github.com/abrt/abrt/blob/master/src/daemon/rpm.c#L184

        if (rpmtsSetRootDir(*ts, rootdir_or_NULL) != 0)
        {
            rpmtsFree(*ts);
            return -1;
        }

And so on.

There are other automatic crash analysis scripts, I believe systemd
also has one - I haven't looked at it all.

WORKAROUND

I highly recommend setting `sysctl -w kern.core_pattern=core`.

EXPLOITATION

Two demonstration exploits are attached.

The file `newpid.c` should produce a root shell on Fedora 20 or Ubuntu
by invoking the crash handler inside an unprivileged chroot (possible
since kernel 3.8).

 $ gcc -static newpid.c
 $ ./a.out
 uid=0(root) gid=0(root) groups=0(root)
 sh-4.3# exit
 exit

The file `raceabrt.c` should make you the owner of any file on Fedora
by racing the Abrt report creation.

 $ cat /etc/fedora-release
 Fedora release 21 (Twenty One)
 $ ./a.out /etc/passwd
 Detected ccpp-2015-04-13-17:40:31-5506.new, attempting to race...
   [ wait a few minutes ]
   exploit successful...
 -rw-r--r--. 1 taviso abrt 2421 Apr 13 11:15 /etc/passwd

In case it isn't obvious, you can then give yourself uid zero.

 $ getent passwd taviso
 taviso:x:1000:1000:Tavis Ormandy:/home/taviso:/bin/bash
 $ vi /etc/passwd
 $ getent passwd taviso
 taviso:x:0:0:Tavis Ormandy:/home/taviso:/bin/bash
 $ su taviso
 Password:
 # id
 uid=0(root) gid=0(root) groups=0(root)
 exit

REFERENCES

[1] https://code.launchpad.net/~stgraber/apport/pidns-support/+merge/200893
[2] https://github.com/abrt/abrt/pull/810
[3] http://man7.org/linux/man-pages/man7/user_namespaces.7.html

CREDIT

Tavis Ormandy, Google Project Zero.

View attachment "raceabrt.c" of type "text/x-csrc" (6519 bytes)

View attachment "newpid.c" of type "text/x-csrc" (5061 bytes)
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