Re: [PATCH v2] perf: Allow restricted kernel breakpoints on user addresses

[Marco Elver]

On Mon, 30 Jan 2023 at 11:46, Mark Rutland <mark.rutland@xxxxxxx> wrote:
[...]
> > This again feels like a deficiency with access_ok(). Is there a better
> > primitive than access_ok(), or can we have something that gives us the
> > guarantee that whatever it says is "ok" is a userspace address?
>
> I don't think so, since this is contextual and temporal -- a helper can't give
> a single correct answert in all cases because it could change.

That's fair, but unfortunate. Just curious: would
copy_from_user_nofault() reliably fail if it tries to access one of
those mappings but where access_ok() said "ok"?

Though that would probably restrict us to only creating watchpoints
for addresses that are actually mapped in the task.

> In the cases we switch to another mapping, we could try to ensure that we
> enable/disable potentially unsafe watchpoints/breakpoints.

That seems it'd be too hard to reason that it's 100% safe, everywhere,
on every arch. I'm still convinced we can prohibit creation of such
watchpoints in the first place, but need something other than
access_ok().

> Taking a look at arm64, our idmap code might actually be ok, since we usually
> mask all the DAIF bits (and the 'D' or 'Debug' bit masks HW
> breakpoints/watchpoints). For EFI we largely switch to another thread (but not
> always), so that would need some auditing.
>
> So if this only needs to work in per-task mode rather than system-wide mode, I
> reckon we can have some save/restore logic around those special cases where we
> transiently install a mapping, which would protect us.

It should only work in per-task mode.

> For the threads that run with special mappings in the low half, I'm not sure
> what to do. If we've ruled out system-wide monitoring I believe those would be
> protected from unprivileged users.

Can the task actually access those special mappings, or is it only
accessible by the kernel?

Thanks,
-- Marco