Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()

[Dan Williams]

On Wed, May 2, 2018 at 9:19 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Tue, May 1, 2018 at 8:34 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx>
> wrote:
>
>> On Tue, May 1, 2018 at 8:22 PM Dan Williams <dan.j.williams@xxxxxxxxx>
>> wrote:
>
>> > All that to say that having a typical RAM page covering poisoned pmem
>> > would complicate the 'clear badblocks' implementation.
>
>> Ugh, ok.
>
>> I guess the good news is that your patches aren't so big, and don't really
>> affect anything else.
>
>
> I pondered this a bit.  Doing better might be a big pain in the arse.  The
> interesting case is where ordinary kernel code (memcpy, plain old memory
> operands, etc) access faulty pmem.  This means that there's no extable
> entry around.  If we actually try to recover, we have a few problems:
>
>   - We can't sanely skip the instruction without causing random errors.
>
>   - If the access was through the kernel direct map, then we could plausibly
> remap a different page in place of the faulty page.  The problem is that,
> if the page is *writable* and we share it between more than one faulty
> page, then we're enabling a giant information leak.  But we still need to
> figure out how we're supposed to invalidate the old mapping from a random,
> potentially atomic context.
>
>   - If the access is through kmap or similar, then we're talking about
> modifying a PTE out from under kernel code that really isn't expecting us
> to modify it.
>
>   - How are we supposed to signal the process or fail a syscall?  The fault
> could have come from interrupt context, softirq context, kernel thread
> context, etc, and figuring out who's to blame seems quite awkward and
> fragile.
>
> All that being said, I suspect that we still have issues even with accesses
> to user VAs that are protected by extable entries.  The whole #MC mechanism
> is a supremely shitty interface for recoverable errors (especially on
> Intel), and I'm a bit scared of what happens if the offending access is,
> say, inside a perf NMI.
>
> Dan, is there any chance you could put some pressure on the architecture
> folks to invent an entirely new, less shitty way to tell the OS about
> recoverable memory errors?  And to make it testable by normal people?
> Needing big metal EINJ hardware to test the house of cards that is #MC is
> just awful and means that there are few enough kernel developers that are
> actually able to test that I can probably count them on one hand.  And I'm
> not one of them...

I feel this testing pain too. The EINJ facility is not ubiquitous
which is why I punted and wrote patch 6 to unit test this. You're
right that does not scale for all the potential places we'd like to be
able to safely handle memory errors in the kernel.