Re: [PATCH v13 1/3] syscall_user_dispatch: helper function to operate on given task

[Gregory Price <gregory.price@memverge.com>]

On Tue, Mar 21, 2023 at 04:41:37PM +0100, Thomas Gleixner wrote:
> Gregory!
> 
> On Wed, Mar 01 2023 at 15:58, Gregory Price wrote:
> > +static int task_set_syscall_user_dispatch(struct task_struct *task, unsigned long mode,
> > +					  unsigned long offset, unsigned long len,
> > +					  char __user *selector)
> >  {
> >  	switch (mode) {
> >  	case PR_SYS_DISPATCH_OFF:
>         ...
> 
> 	case PR_SYS_DISPATCH_ON:
> 		if (selector && !access_ok(selector, sizeof(*selector)))
> 			return -EFAULT;
> 
> I'm not seing how this can work on ARM64 when user pointer tagging is
> enabled in the tracee, but not in the tracer. In such a case, if the
> pointer is tagged, access_ok() will fail because access_ok() wont untag
> it.
> 
> Thanks,
> 
>         tglx

I see that untagged_addr(x) is available to clear tags, I don't see an
immediate issues with converting to:

!access_ok(untagged_addr(selector), sizeof(*selector))

In both the tracee calling the prctl interface and the tracer calling
the ptrace interface the tag will be cleared, which appears to be the
intended effect.  Just want a sanity check before i push it through, as
I'm not overly familiar with the ARM/tagging ecosystem.

Seems reasoanble that this change should live with this commit, so i'll
plan to squash and push it up if the change is reasonable.

Thanks for your input
~Gregory