linux-kernel - Re: [PATCH] mm, kasan: don't poison boot memory

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Message-ID: <d11bf144-669b-0fe1-4fa4-001a014db32a@oracle.com>
Date:   Thu, 18 Feb 2021 19:06:33 -0500
From:   George Kennedy <george.kennedy@...cle.com>
To:     David Hildenbrand <david@...hat.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Konrad Rzeszutek Wilk <konrad@...nok.org>
Cc:     Will Deacon <will.deacon@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Christoph Hellwig <hch@...radead.org>,
        kasan-dev@...glegroups.com, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dhaval Giani <dhaval.giani@...cle.com>
Subject: Re: [PATCH] mm, kasan: don't poison boot memory



On 2/18/2021 3:55 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 <andreyknvl@...gle.com>
>> 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 :)

Since David's patch we're having trouble with the iBFT ACPI table, which 
is mapped in via kmap() - see acpi_map() in "drivers/acpi/osl.c". KASAN 
detects that it is being used after free when ibft_init() accesses the 
iBFT table, but as of yet we can't find where it get's freed (we've 
instrumented calls to kunmap()).

Thank you,
George