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

[George Kennedy <george.kennedy@oracle.com>]

On 2/19/2021 11:45 AM, George Kennedy wrote:
>
>
> 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:

Cleaned up version:

diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index 0418feb..8f0a8e7 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -287,9 +287,12 @@ 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));
+        SetPageReserved(page);
+        return (void __iomem __force *)kmap(page);
      } else
          return acpi_os_ioremap(pg_off, pg_sz);
  }
@@ -299,9 +302,12 @@ static void acpi_unmap(acpi_physical_address 
pg_off, void __iomem *vaddr)
      unsigned long pfn;

      pfn = pg_off >> PAGE_SHIFT;
-    if (should_use_kmap(pfn))
-        kunmap(pfn_to_page(pfn));
-    else
+    if (should_use_kmap(pfn)) {
+        struct page *page = pfn_to_page(pfn);
+
+        ClearPageReserved(page);
+        kunmap(page);
+    } else
          iounmap(vaddr);
  }

David, the above works, but wondering why it is now necessary. kunmap() 
is not hit. What other ways could a page mapped via kmap() be unmapped?

Thank you,
George

>
> 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
>