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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