Messages in this thread | | | Subject | Re: user pointers and race conditions | From | Arjan van de Ven <> | Date | Thu, 17 May 2007 05:26:56 -0700 |
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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 > _________________________________________________________________ > Download Messenger. Start an i’m conversation. Support a cause. Join now. > http://im.live.com/messenger/im/home/?source=TAGWL_MAY07- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- if you want to mail me at work (you don't), use arjan (at) linux.intel.com Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org
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