Re: user pointers and race conditions

[Al Viro <viro@ftp.linux.org.uk>]

On Wed, May 16, 2007 at 10:56:22PM -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)))

... and there's your bug - direct access to userland data.  The normal
variant is to use accessors (get_user() or copy_from_user()) to fetch
the value of stp->u.  At which point races of the kind you mentioned
take an obviously dumb code (explicitly copying the same struct from
userland _twice_).

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