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[2003:cb:c70c:cd00:c139:924e:3595:3b5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43186bdbb01sm10999135e9.20.2024.10.23.02.29.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Oct 2024 02:29:53 -0700 (PDT) Message-ID: <1768ef5d-7289-4d2b-ae02-f5d2a20d5320@redhat.com> Date: Wed, 23 Oct 2024 11:29:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/5] implement lightweight guard pages To: Lorenzo Stoakes Cc: 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, elver@google.com, 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> 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: B9F00A002B X-Stat-Signature: 9i558w6i1nt1bm5fujs1tw5fdbqcbppo X-Rspam-User: X-HE-Tag: 1729675786-169730 X-HE-Meta: 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 x2eU0WAQ vwY3VTSlebDWEx8f1/DTQXtwo7R1h+FHxgbeGntKuv2DkQM5Ozf+aB9NpXXm//NSl1MwgrX+pBO77HBy2/tV2N5TDBiCrwXrFaN9wByR+vIKWuYaYEsxNN0vh6dn/DHyWCp+FrkAeKg0AIvX/9TQ5N+kCcaj7ab6Hg7WgasAl1IyAF/715rIAANzZLApjoFdRrl4wzeJfbxLT+KqdQNz9CqlTilyUUhHCE+U4ETRAKwX9gdgiXCCPYDVsJJfH/Scd86eKFEKf4uZ16MelGJlZ7LkiVj4YPTCRRRfTw+6XLNfvWM566VWMkK2II6Z4svaG5hJjS2r26pzoitJCliCSj3ULECJyDWy6J3NPt16Md8YsAssbs3zoNvQ2uhyVzdqi57A9cwheF5R97V9hdfcOJBGZmg== 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 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. -- Cheers, David / dhildenb