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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 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 38D3118001A X-Stat-Signature: ge6o3mafwi9wwrufx6osog8y3aqzai34 X-Rspam-User: X-HE-Tag: 1729683360-559051 X-HE-Meta: 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 ZyzRxFX8 iYkxOPI35L0WkNMlnG+C8YdisAuuTCJKAjT/H42xkB9PoeTRO/3u17Ijzz7i9igvbR3wUA+PcUD2xV9K4FqwwJmZJV8RrcUowhFVHIoRHjzTP7s5VgDzFDpG5VqI7vv2rRsGSv33bwAIhGBtvTvaQjiCCLZ/6ZnHf4DuxVUpPXPVsT31aPVPAJxysj19RZEeemdAGbmaWQszWC2Y5xz5hvJLHpZSmZvx/nsveCfBjaxfXA2UvHs2GRACR3KYDzB1SGCaLbYY6k+XLFVXt3sa96ip9ervp1rqWMmiQUqTAAgKoEK8me9DtQ9MEIHmfisV7uwICyJFIgUevcpM7v8zJITSXGt2UGk6MpUzByGekdFh2JsiVDq9AdkYuEq0iJDUJl7TITgYrWCE5PiR1BJRD9Uj3lT9U/i8PpIqFJkdwuapwxGM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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