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

From: David Hildenbrand <david@redhat.com>
To: Andrey Konovalov <andreyknvl@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	George Kennedy <george.kennedy@oracle.com>,
	Konrad Rzeszutek Wilk <konrad@darnok.org>
Cc: Will Deacon <will.deacon@arm.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>,
	Peter Collingbourne <pcc@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Branislav Rankov <Branislav.Rankov@arm.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	kasan-dev@googlegroups.com, linux-arm-kernel@lists.infradead.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm, kasan: don't poison boot memory
Date: Thu, 18 Feb 2021 09:55:07 +0100	[thread overview]
Message-ID: <e58cbb53-5f5b-42ae-54a0-e3e1b76ad271@redhat.com> (raw)
In-Reply-To: <487751e1ccec8fcd32e25a06ce000617e96d7ae1.1613595269.git.andreyknvl@google.com>

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@google.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 :)

-- 
Thanks,

David / dhildenb