Re: user pointers and race conditions

[Arjan van de Ven]

On Wed, 2007-05-16 at 22:56 -0600, sk b wrote:
> Hello,
> 
> I'm wondering whether there is an exploitable TOCTTOU race condition in the way user pointers are handled in the kernel. Consider the following code:
> 
> 1: struct st { int *u; };
> 2: void syscall(struct st * stp) {
> 3:        if (!access_ok(VERIFY_READ,stp,sizeof(struct st)))
> 4:                return;
> 5:        if (!access_ok(VERIFY_WRITE,stp->u,sizeof(int)))
> 6:                return;

this line is invalid; you are not allowed to dereference userspace
memory directly. You need to call copy_from_user() on stp first, and
then use the copy. 
> 7:        foo();   //user app writes a kernel address to stp->u
> 8:        *(stp->u) = 0;
> 9:}
> 
> Suppose syscall is some system call and, thus, stp and stp->u are user pointers. The function checks the stp and stp->u pointers using the access_ok macro on lines 3 and 5. Also suppose that the call to foo on line 7  takes a non-trivial amount of time to execute. During the time it takes foo to execute, the user application writes a kernel address to stp->u. Note that this write occurs after the check on line 5. Then, on line 8, the kernel writes to stp->u which contains a kernel address. So, the user application could force the kernel to overwrite itself. Is it possible to exploit this race condition? If so, does Sparse check for this?
> 
> -SKB
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