linux-kernel - Re: [PATCH v2 0/5] implement lightweight guard pages

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Message-ID: <2c674241-7e40-46a0-9506-85ef37e3578a@redhat.com>
Date: Wed, 23 Oct 2024 13:36:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Marco Elver <elver@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Dmitry Vyukov <dvyukov@...gle.com>,
 fw@...eb.enyo.de, James.Bottomley@...senpartnership.com,
 Liam.Howlett@...cle.com, akpm@...ux-foundation.org, arnd@...db.de,
 brauner@...nel.org, chris@...kel.net, deller@....de, hch@...radead.org,
 ink@...assic.park.msu.ru, jannh@...gle.com, jcmvbkbc@...il.com,
 jeffxu@...omium.org, jhubbard@...dia.com, linux-alpha@...r.kernel.org,
 linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mm@...ck.org,
 linux-parisc@...r.kernel.org, mattst88@...il.com, muchun.song@...ux.dev,
 paulmck@...nel.org, richard.henderson@...aro.org, shuah@...nel.org,
 sidhartha.kumar@...cle.com, surenb@...gle.com, tsbogend@...ha.franken.de,
 willy@...radead.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 0/5] implement lightweight guard pages

On 23.10.24 13:31, Marco Elver wrote:
> On Wed, 23 Oct 2024 at 11:29, David Hildenbrand <david@...hat.com> wrote:
>>
>> On 23.10.24 11:18, Lorenzo Stoakes wrote:
>>> On Wed, Oct 23, 2024 at 11:13:47AM +0200, David Hildenbrand wrote:
>>>> On 23.10.24 11:06, Vlastimil Babka wrote:
>>>>> On 10/23/24 10:56, Dmitry Vyukov wrote:
>>>>>>>
>>>>>>> Overall while I sympathise with this, it feels dangerous and a pretty major
>>>>>>> change, because there'll be something somewhere that will break because it
>>>>>>> expects faults to be swallowed that we no longer do swallow.
>>>>>>>
>>>>>>> So I'd say it'd be something we should defer, but of course it's a highly
>>>>>>> user-facing change so how easy that would be I don't know.
>>>>>>>
>>>>>>> But I definitely don't think a 'introduce the ability to do cheap PROT_NONE
>>>>>>> guards' series is the place to also fundmentally change how user access
>>>>>>> page faults are handled within the kernel :)
>>>>>>
>>>>>> Will delivering signals on kernel access be a backwards compatible
>>>>>> change? Or will we need a different API? MADV_GUARD_POISON_KERNEL?
>>>>>> It's just somewhat painful to detect/update all userspace if we add
>>>>>> this feature in future. Can we say signal delivery on kernel accesses
>>>>>> is unspecified?
>>>>>
>>>>> Would adding signal delivery to guard PTEs only help enough the ASAN etc
>>>>> usecase? Wouldn't it be instead possible to add some prctl to opt-in the
>>>>> whole ASANized process to deliver all existing segfaults as signals instead
>>>>> of -EFAULT ?
>>>>
>>>> Not sure if it is an "instead", you might have to deliver the signal in
>>>> addition to letting the syscall fail (not that I would be an expert on
>>>> signal delivery :D ).
>>>>
>>>> prctl sounds better, or some way to configure the behavior on VMA ranges;
>>>> otherwise we would need yet another marker, which is not the end of the
>>>> world but would make it slightly more confusing.
>>>>
>>>
>>> Yeah prctl() sounds sensible, and since we are explicitly adding a marker
>>> for guard pages here we can do this as a follow up too without breaking any
>>> userland expectations, i.e. 'new feature to make guard pages signal' is not
>>> going to contradict the default behaviour.
>>>
>>> So all makes sense to me, but I do think best as a follow up! :)
>>
>> Yeah, fully agreed. And my gut feeling is that it might not be that easy
>> ... :)
>>
>> In the end, what we want is *some* notification that a guard PTE was
>> accessed. Likely the notification must not necessarily completely
>> synchronous (although it would be ideal) and it must not be a signal.
>>
>> Maybe having a different way to obtain that information from user space
>> would work.
> 
> For bug detection tools (like GWP-ASan [1]) it's essential to have
> useful stack traces. As such, having this signal be synchronous would
> be more useful. I don't see how one could get a useful stack trace (or
> other information like what's stashed away in ucontext like CPU
> registers) if this were asynchronous.

Yes, I know. But it would be better than not getting *any* notification 
except of some syscalls simply failing with -EFAULT, and not having an 
idea which address was even accessed.

Maybe the signal injection is easier than I think, but I somehow doubt 
it ...

-- 
Cheers,

David / dhildenb