Vulnerability: The compat_sys_recvmmsg function in net/compat.c in the Linux kernel before 3.13.2, when CONFIG_X86_X32 is enabled, allows local users to gain privileges via a recvmmsg system call with a crafted timeout pointer parameter.
Vulnerability ID: CVE-2014-0038
CWE: CWE-20
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2014-02-06
File: net/compat.c
Commit message: x86, x32: Correct invalid use of user timespec in the kernel

The x32 case for the recvmsg() timout handling is broken:

  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 bug was introduced by commit ee4fa23c4bfc ("compat: Use
COMPAT_USE_64BIT_TIME in net/compat.c") 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.

Other uses of COMPAT_USE_64BIT_TIME seem fine.

This addresses CVE-2014-0038.

Signed-off-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Commit author: PaX Team

Vulnerable version (before fix):
 asmlinkage long compat_sys_recvmmsg(int fd, struct compat_mmsghdr __user *mmsg, 	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)) 		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;  	return datagrams;

Fixed version (after fix):
 asmlinkage long compat_sys_recvmmsg(int fd, struct compat_mmsghdr __user *mmsg, 	if (flags & MSG_CMSG_COMPAT) 		return -EINVAL;  	if (timeout == NULL) 		return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen, 				      flags | MSG_CMSG_COMPAT, NULL);  	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 && compat_put_timespec(&ktspec, timeout)) 		datagrams = -EFAULT;  	return datagrams;

Patch:
patch: @@ -780,21 +780,16 @@ asmlinkage long compat_sys_recvmmsg(int fd, struct compat_mmsghdr __user *mmsg,
 	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;
path: net/compat.c