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[2003:cb:c70c:cd00:c139:924e:3595:3b5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37ee0a5b7afsm8711333f8f.59.2024.10.23.04.36.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Oct 2024 04:36:13 -0700 (PDT) Message-ID: <2c674241-7e40-46a0-9506-85ef37e3578a@redhat.com> Date: Wed, 23 Oct 2024 13:36:10 +0200 Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/5] implement lightweight guard pages To: Marco Elver Cc: Lorenzo Stoakes , Vlastimil Babka , Dmitry Vyukov , fw@deneb.enyo.de, James.Bottomley@hansenpartnership.com, Liam.Howlett@oracle.com, akpm@linux-foundation.org, arnd@arndb.de, brauner@kernel.org, chris@zankel.net, deller@gmx.de, hch@infradead.org, ink@jurassic.park.msu.ru, jannh@google.com, jcmvbkbc@gmail.com, jeffxu@chromium.org, jhubbard@nvidia.com, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, mattst88@gmail.com, muchun.song@linux.dev, paulmck@kernel.org, richard.henderson@linaro.org, shuah@kernel.org, sidhartha.kumar@oracle.com, surenb@google.com, tsbogend@alpha.franken.de, willy@infradead.org, Linus Torvalds References: <87a5eysmj1.fsf@mid.deneb.enyo.de> <20241023062417.3862170-1-dvyukov@google.com> <8471d7b1-576b-41a6-91fb-1c9baae8c540@redhat.com> <5a3d3bc8-60db-46d0-b689-9aeabcdb8eab@lucifer.local> <1768ef5d-7289-4d2b-ae02-f5d2a20d5320@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 23.10.24 13:31, Marco Elver wrote: > On Wed, 23 Oct 2024 at 11:29, David Hildenbrand 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