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

[Andrey Konovalov]

On Fri, Feb 19, 2021 at 1:06 AM George Kennedy
<george.kennedy@xxxxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxx>
> >> Change-Id: Iae6b1e4bb8216955ffc14af255a7eaaa6f35324d
> >
> > Not sure this is the right thing to do, see
> >
> > https://lkml.kernel.org/r/bcf8925d-0949-3fe1-baa8-cc536c529860@xxxxxxxxxx
> >
> > 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()).

Maybe it doesn't get freed, but what you see is a wild or a large
out-of-bounds access. Since KASAN marks all memory as freed during the
memblock->page_alloc transition, such bugs can manifest as
use-after-frees.