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Thu, 18 Feb 2021 14:46:25 -0500 X-MC-Unique: 55MMnnLmMF2NSFx3LYxoGg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B8721C297; Thu, 18 Feb 2021 19:46:22 +0000 (UTC) Received: from [10.36.114.59] (ovpn-114-59.ams2.redhat.com [10.36.114.59]) by smtp.corp.redhat.com (Postfix) with ESMTP id AB0E410016DB; Thu, 18 Feb 2021 19:46:18 +0000 (UTC) Subject: Re: [PATCH] mm, kasan: don't poison boot memory To: Andrey Konovalov References: <487751e1ccec8fcd32e25a06ce000617e96d7ae1.1613595269.git.andreyknvl@google.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <509c1c80-bb2c-0c5c-ffa3-939ca40d2646@redhat.com> Date: Thu, 18 Feb 2021 20:46:17 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210218_144630_684241_D081C61C X-CRM114-Status: GOOD ( 27.36 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Linux ARM , Marco Elver , Linux Memory Management List , Catalin Marinas , Kevin Brodsky , Will Deacon , Branislav Rankov , kasan-dev , LKML , Christoph Hellwig , George Kennedy , Alexander Potapenko , Evgenii Stepanov , Andrey Ryabinin , Konrad Rzeszutek Wilk , Andrew Morton , Vincenzo Frascino , Peter Collingbourne , Dmitry Vyukov Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 18.02.21 20:40, Andrey Konovalov wrote: > On Thu, Feb 18, 2021 at 9:55 AM David Hildenbrand wrote: >> >> On 17.02.21 21:56, Andrey Konovalov wrote: >>> During boot, all non-reserved memblock memory is exposed to the buddy >>> allocator. Poisoning all that memory with KASAN lengthens boot time, >>> especially on systems with large amount of RAM. This patch makes >>> page_alloc to not call kasan_free_pages() on all new memory. >>> >>> __free_pages_core() is used when exposing fresh memory during system >>> boot and when onlining memory during hotplug. This patch adds a new >>> FPI_SKIP_KASAN_POISON flag and passes it to __free_pages_ok() through >>> free_pages_prepare() from __free_pages_core(). >>> >>> This has little impact on KASAN memory tracking. >>> >>> Assuming that there are no references to newly exposed pages before they >>> are ever allocated, there won't be any intended (but buggy) accesses to >>> that memory that KASAN would normally detect. >>> >>> However, with this patch, KASAN stops detecting wild and large >>> out-of-bounds accesses that happen to land on a fresh memory page that >>> was never allocated. This is taken as an acceptable trade-off. >>> >>> All memory allocated normally when the boot is over keeps getting >>> poisoned as usual. >>> >>> Signed-off-by: Andrey Konovalov >>> Change-Id: Iae6b1e4bb8216955ffc14af255a7eaaa6f35324d >> >> Not sure this is the right thing to do, see >> >> https://lkml.kernel.org/r/bcf8925d-0949-3fe1-baa8-cc536c529860@oracle.com >> >> Reversing the order in which memory gets allocated + used during boot >> (in a patch by me) might have revealed an invalid memory access during boot. >> >> I suspect that that issue would no longer get detected with your patch, >> as the invalid memory access would simply not get detected. Now, I >> cannot prove that :) > > This looks like a good example. > > Ok, what we can do is: > > 1. For KASAN_GENERIC: leave everything as is to be able to detect > these boot-time bugs. > > 2. For KASAN_SW_TAGS: remove boot-time poisoning via > kasan_free_pages(), but use the "invalid" tag as the default shadow > value. The end result should be the same: bad accesses will be > detected. For unallocated memory as it has the default "invalid" tag, > and for allocated memory as it's poisoned properly when > allocated/freed. > > 3. For KASAN_HW_TAGS: just remove boot-time poisoning via > kasan_free_pages(). As the memory tags have a random unspecified > value, we'll still have a 15/16 chance to detect a memory corruption. > > This also makes sense from the performance perspective: KASAN_GENERIC > isn't meant to be running in production, so having a larger perf > impact is acceptable. The other two modes will be faster. Sounds in principle sane to me. Side note: I am not sure if anybody runs KASAN in production. Memory is expensive. Feel free to prove me wrong, I'd be very interest in actual users. -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel