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

[George Kennedy]

On 2/18/2021 7:09 PM, Andrey Konovalov wrote:
> 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.

It gets freed and re-used. By the time the iBFT table is accessed by 
ibft_init() the page has been over-written.

Setting page flags like the following before the call to kmap() prevents 
the iBFT table page from being freed:

diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index 0418feb..41c1bbd 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -287,9 +287,14 @@ static void __iomem *acpi_map(acpi_physical_address 
pg_off, unsigned long pg_sz)

        pfn = pg_off >> PAGE_SHIFT;
        if (should_use_kmap(pfn)) {
+               struct page *page =  pfn_to_page(pfn);
+
                if (pg_sz > PAGE_SIZE)
                        return NULL;
-               return (void __iomem __force *)kmap(pfn_to_page(pfn));
+
+               page->flags |= ((1UL << PG_unevictable) | (1UL << 
PG_reserved) | (1UL << PG_locked));
+
+               return (void __iomem __force *)kmap(page);
        } else
                return acpi_os_ioremap(pg_off, pg_sz);
 }

Just not sure of the correct way to set the page flags.

George