Re: [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup()

[Andy Lutomirski <luto@kernel.org>]

On Mon, Jan 11, 2021 at 1:45 PM Tony Luck <tony.luck@intel.com> wrote:
>
> Linux can now recover from machine checks where kernel code is
> doing get_user() to access application memory. But there isn't
> a way to distinguish whether get_user() failed because of a page
> fault or a machine check.
>
> Thus there is a problem if any kernel code thinks it can retry
> an access after doing something that would fix the page fault.
>
> One such example (I'm sure there are more) is in futex_wait_setup()
> where an attempt to read the futex with page faults disabled. Then
> a retry (after dropping a lock so page faults are safe):
>
>
>         ret = get_futex_value_locked(&uval, uaddr);
>
>         if (ret) {
>                 queue_unlock(*hb);
>
>                 ret = get_user(uval, uaddr);
>
> It would be good to avoid deliberately taking a second machine
> check (especially as the recovery code does really bad things
> and ends up in an infinite loop!).
>
> V2 (thanks to feedback from PeterZ) fixes this by changing get_user() to
> return -ENXIO ("No such device or address") for the case where a machine
> check occurred. Peter left it open which error code to use (suggesting
> "-EMEMERR or whatever name we come up with"). I think the existing ENXIO
> error code seems appropriate (the address being accessed has effectively
> gone away). But I don't have a strong attachment if anyone thinks we
> need a new code.
>
> Callers can check for ENXIO in paths where the access would be
> retried so they can avoid a second machine check.
>

I don't love this -- I'm concerned that there will be some code path
that expects a failing get_user() to return -EFAULT, not -ENXIO.
Also, get_user() *can* return -EFAULT when it hits bad memory even
with your patch if the recovery code manages to yank the PTE before
get_user().

So I tend to think that the machine check code should arrange to
survive some reasonable number of duplicate machine checks.