Messages in this thread Patch in this message | | | Subject | Re: [PATCH] mm, kasan: don't poison boot memory | From | George Kennedy <> | Date | Fri, 19 Feb 2021 11:45:50 -0500 |
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On 2/18/2021 7:09 PM, Andrey Konovalov wrote: > On Fri, Feb 19, 2021 at 1:06 AM George Kennedy > <george.kennedy@oracle.com> 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@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 :) >> 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
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