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

From: Konrad Rzeszutek Wilk
Date: Mon Feb 22 2021 - 12:41:55 EST

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On Mon, Feb 22, 2021 at 05:39:29PM +0100, David Hildenbrand wrote:
> On 22.02.21 17:13, David Hildenbrand wrote:
> > On 22.02.21 16:13, George Kennedy wrote:
> > >
> > >
> > > On 2/22/2021 4:52 AM, David Hildenbrand wrote:
> > > > On 20.02.21 00:04, George Kennedy wrote:
> > > > >
> > > > >
> > > > > 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@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:
> > > > >
> > > > > 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?
> > > > >
> > > >
> > > > Let me look into the code ... I have little experience with ACPI
> > > > details, so bear with me.
> > > >
> > > > I assume that acpi_map()/acpi_unmap() map some firmware blob that is
> > > > provided via firmware/bios/... to us.
> > > >
> > > > should_use_kmap() tells us whether
> > > > a) we have a "struct page" and should kmap() that one
> > > > b) we don't have a "struct page" and should ioremap.
> > > >
> > > > As it is a blob, the firmware should always reserve that memory region
> > > > via memblock (e.g., memblock_reserve()), such that we either
> > > > 1) don't create a memmap ("struct page") at all (-> case b) )
> > > > 2) if we have to create e memmap, we mark the page PG_reserved and
> > > >    *never* expose it to the buddy (-> case a) )
> > > >
> > > >
> > > > Are you telling me that in this case we might have a memmap for the HW
> > > > blob that is *not* PG_reserved? In that case it most probably got
> > > > exposed to the buddy where it can happily get allocated/freed.
> > > >
> > > > The latent BUG would be that that blob gets exposed to the system like
> > > > ordinary RAM, and not reserved via memblock early during boot.
> > > > Assuming that blob has a low physical address, with my patch it will
> > > > get allocated/used a lot earlier - which would mean we trigger this
> > > > latent BUG now more easily.
> > > >
> > > > There have been similar latent BUGs on ARM boards that my patch
> > > > discovered where special RAM regions did not get marked as reserved
> > > > via the device tree properly.
> > > >
> > > > Now, this is just a wild guess :) Can you dump the page when mapping
> > > > (before PageReserved()) and when unmapping, to see what the state of
> > > > that memmap is?
> > >
> > > Thank you David for the explanation and your help on this,
> > >
> > > dump_page() before PageReserved and before kmap() in the above patch:
> > >
> > > [    1.116480] ACPI: Core revision 20201113
> > > [    1.117628] XXX acpi_map: about to call kmap()...
> > > [    1.118561] page:ffffea0002f914c0 refcount:0 mapcount:0
> > > mapping:0000000000000000 index:0x0 pfn:0xbe453
> > > [    1.120381] flags: 0xfffffc0000000()
> > > [    1.121116] raw: 000fffffc0000000 ffffea0002f914c8 ffffea0002f914c8
> > > 0000000000000000
> > > [    1.122638] raw: 0000000000000000 0000000000000000 00000000ffffffff
> > > 0000000000000000
> > > [    1.124146] page dumped because: acpi_map pre SetPageReserved
> > >
> > > I also added dump_page() before unmapping, but it is not hit. The
> > > following for the same pfn now shows up I believe as a result of setting
> > > PageReserved:
> > >
> > > [   28.098208] BUG:Bad page state in process mo dprobe pfn:be453
> > > [   28.098394] page:ffffea0002f914c0 refcount:0 mapcount:0
> > > mapping:0000000000000000 index:0x1 pfn:0xbe453
> > > [   28.098394] flags: 0xfffffc0001000(reserved)
> > > [   28.098394] raw: 000fffffc0001000 dead000000000100 dead000000000122
> > > 0000000000000000
> > > [   28.098394] raw: 0000000000000001 0000000000000000 00000000ffffffff
> > > 0000000000000000
> > > [   28.098394] page dumped because: PAGE_FLAGS_CHECK_AT_PREP flag(s) set
> > > [   28.098394] page_owner info is not present (never set?)
> > > [   28.098394] Modules linked in:
> > > [   28.098394] CPU: 2 PID: 204 Comm: modprobe Not tainted 5.11.0-3dbd5e3 #66
> > > [   28.098394] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> > > BIOS 0.0.0 02/06/2015
> > > [   28.098394] Call Trace:
> > > [   28.098394] dump_stack+0xdb/0x120
> > > [   28.098394] bad_page.cold.108+0xc6/0xcb
> > > [   28.098394] check_new_page_bad+0x47/0xa0
> > > [   28.098394] get_page_from_freelist+0x30cd/0x5730
> > > [   28.098394] ? __isolate_free_page+0x4f0/0x4f0
> > > [   28.098394] ? init_object+0x7e/0x90
> > > [   28.098394] __alloc_pages_nodemask+0x2d8/0x650
> > > [   28.098394] ? write_comp_data+0x2f/0x90
> > > [   28.098394] ? __alloc_pages_slowpath.constprop.103+0x2110/0x2110
> > > [   28.098394] ? __sanitizer_cov_trace_pc+0x21/0x50
> > > [   28.098394] alloc_pages_vma+0xe2/0x560
> > > [   28.098394] do_fault+0x194/0x12c0
> > > [   28.098394] ? write_comp_data+0x2f/0x90
> > > [   28.098394] __handle_mm_fault+0x1650/0x26c0
> > > [   28.098394] ? copy_page_range+0x1350/0x1350
> > > [   28.098394] ? write_comp_data+0x2f/0x90
> > > [   28.098394] ? write_comp_data+0x2f/0x90
> > > [   28.098394] handle_mm_fault+0x1f9/0x810
> > > [   28.098394] ? write_comp_data+0x2f/0x90
> > > [   28.098394] do_user_addr_fault+0x6f7/0xca0
> > > [   28.098394] exc_page_fault+0xaf/0x1a0
> > > [   28.098394] asm_exc_page_fault+0x1e/0x30
> > > [   28.098394] RIP: 0010:__clear_user+0x30/0x60
> >
> > I think the PAGE_FLAGS_CHECK_AT_PREP check in this instance means that
> > someone is trying to allocate that page with the PG_reserved bit set.
> > This means that the page actually was exposed to the buddy.
> >
> > However, when you SetPageReserved(), I don't think that PG_buddy is set
> > and the refcount is 0. That could indicate that the page is on the buddy
> > PCP list. Could be that it is getting reused a couple of times.
> >
> > The PFN 0xbe453 looks a little strange, though. Do we expect ACPI tables
> > close to 3 GiB ? No idea. Could it be that you are trying to map a wrong
> > table? Just a guess.

Nah, ACPI MADT enumerates the table and that is the proper location of it.
>
> ... but I assume ibft_check_device() would bail out on an invalid checksum.
> So the question is, why is this page not properly marked as reserved
> already.

The ibft_check_device ends up being called as module way way after the
kernel has cleaned the memory.

The funny thing about iBFT is that (it is also mentioned in the spec)
that the table can resize in memory .. or in the ACPI regions (which
have no E820_RAM and are considered "MMIO" regions).

Either place is fine, so it can be in either RAM or MMIO :-(

>
> --
> Thanks,
>
> David / dhildenb
>

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