If you re running a 64bit system, and you ve got users with access to a video device (
/dev/video*), then be sure you update your kernels for
CVE-2010-2963. I ve been slowly making my way through auditing the many uses in the Linux kernel of the
copy_from_user() function, and ran into this vulnerability.
Here s the kernel code from
drivers/media/video/v4l2-compat-ioctl32.c:
static int get_microcode32(struct video_code *kp, struct video_code32 __user *up)
if (!access_ok(VERIFY_READ, up, sizeof(struct video_code32))
copy_from_user(kp->loadwhat, up->loadwhat, sizeof(up->loadwhat))
get_user(kp->datasize, &up->datasize)
copy_from_user(kp->data, up->data, up->datasize))
return -EFAULT;
return 0;
Note that
kp->data is being used as the target for
up->data in the final
copy_from_user() without actually verifying that
kp->data is pointing anywhere safe. Here s the caller of
get_microcode32:
static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
union
struct video_tuner vt;
struct video_code vc;
...
karg;
void __user *up = compat_ptr(arg);
...
switch (cmd)
...
case VIDIOCSMICROCODE:
err = get_microcode32(&karg.vc, up);
...
So, the contents of
up are totally under control of the caller, and the contents of
karg (in our case, the
video_code structure) are
not initialized at all. So, it seems like a call for
VIDIOCSMICROCODE would write
video_code->datasize bytes from
video_code->data into some random kernel address, just causing an Oops, since we don t control what is on the kernel s stack.
But wait, who says we can t control the contents of the kernel s stack? In fact, this compat function makes it extremely easy. Let s look back at the union. Notice the
struct video_tuner? That gets populated from the caller s
up memory via this case of the
switch (cmd) statement:
...
case VIDIOCSTUNER:
case VIDIOCGTUNER:
err = get_video_tuner32(&karg.vt, up);
...
So, to control the kernel stack, we just need to call this
ioctl twice in a row: once to populate the stack via
VIDIOCSTUNER with the contents we want (including the future address for
video_code->data, which starts at the same location as
video_tuner->name[20]), and then again with
VIDIOCSMICROCODE.
Tricks involved here are: the definition of the
VIDIOCSMICROCODE case in the kernel is wrong, and calling the
ioctls without any preparation can trigger other kernel work (memory faults, etc) that may destroy the stack contents. First, we need the real value for the desired case statement. This turns out to be 0 4020761b. Next, we just repeatedly call the setup
ioctl in an attempt to get incidental kernel work out of the way so that our last
ioctl doing the stack preparation will stick, and then we call the buggy
ioctl to trigger the vulnerability.
Since the ioctl already does a multi-byte copy, we can now copy arbitrary lengths of bytes into kernel memory. One method of turning an arbitrary kernel memory write into a privilege escalation is to overwrite a kernel function pointer, and trigger that function. Based on the
exploit for CVE-2010-3081, I opted to overwrite the
security_ops function pointer table. Their use of
msg_queue_msgctl wasn t very good for the general case since it s near the end of the table and its offset would depend on kernel versions. Initially I opted for
getcap, but in the end used
ptrace_traceme, both of which are very near the top the
security_ops structure. (Though I need share credit here with
Dan Rosenberg as we were working together on improving the reliability of the
security_ops overwrite method. He used the same approach for his excellent
RDS exploit.)
Here are the steps for one way of taking an arbitrary kernel memory write and turning it into a root escalation:
- overwrite
security_ops with default_security_ops, which will revert the LSM back to the capabilities-only security operations. This, however, means we can calculate where cap_ptrace_traceme is.
- overwrite
default_security_ops->ptrace_traceme to point to our supplied function that will actually perform the privilege escalation (thanks to Brad Spengler for his code from Enlightenment).
- trigger the function (in this case, call
ptrace(PTRACE_TRACEME, 0, NULL, NULL)).
- restore
default_security_ops->ptrace_traceme to point to cap_ptrace_traceme so the next caller doesn t Oops the system (since userspace memory will be remapped).
Here s the
source for
Vyakarana as seen
running in Enlightenment using
cap_getcap (which is pretty unstable, so you might want to switch it to use
ptrace_traceme), and as a
stand-alone memory writer.
Conclusions: Keep auditing the kernel for more arbitrary writes; I think there are still many left. Reduce the exploitation surface within the kernel itself (which
PaX and grsecurity have been doing for a while now), specifically:
- Block userspace memory access while in kernel mode. This would stop the ability to make the kernel start executing functions that live in userspace a clear privilege violation. This protection would stop the current exploit above, but the exploit could be adjusted to use kernel memory instead.
- Keep function pointers read-only. There is no reason for these function pointer tables (
fops, IDT, security_ops, etc) to be writable. These should all be marked correctly, with inline code exceptions being made for updating the global pointers to those tables, leaving the pointer read-only after it gets set. This would stop this particular exploit above, but there are still plenty more targets.
- Randomize the kernel stack location on a per-syscall basis. This will stop exploits that depend on a stable kernel stack location (as this exploit does).
This is my monthly summary of my Debian related activities. If you re among the people who 
Dpkg work
When I came back from Debconf, I merged my implementation of 
Vacation
That s not so much compared to usual but to my defense I also took 2 weeks of vacation with my family. But somehow even in vacation I can t really forget Debian. Here s my son:
The Scavengers ship, from the Doctor Who Spam Filtered story, 2011:
Such a great ship. Not even remotely made to look aerodynamic.
And to make this almost related to Ubuntu and Debian, here was my command line to remove exif data from the image I took with my phone:
In my 
Buying a ThinkPad?
Don't buy one with