On Thu, Feb 18, 2021 at 9:55 AM David Hildenbrand <david@xxxxxxxxxx> 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 :) 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.
