Messages in this thread | | | Subject | Re: [PATCH] mm, kasan: don't poison boot memory | From | David Hildenbrand <> | Date | Thu, 18 Feb 2021 20:46:17 +0100 |
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On 18.02.21 20:40, Andrey Konovalov wrote: > On Thu, Feb 18, 2021 at 9:55 AM David Hildenbrand <david@redhat.com> 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@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 :) > > 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
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