user pointers and race conditions

[sk b]

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;
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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