Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on tag-based kasan

[Andrey Konovalov <andreyknvl@google.com>]

On Mon, Aug 19, 2019 at 5:03 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Mon, Aug 19, 2019 at 04:05:22PM +0200, Andrey Konovalov wrote:
> > On Mon, Aug 19, 2019 at 3:34 PM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Mon, Aug 19, 2019 at 02:23:48PM +0100, Mark Rutland wrote:
> > > > On Mon, Aug 19, 2019 at 01:56:26PM +0100, Will Deacon wrote:
> > > > > On Mon, Aug 19, 2019 at 07:44:20PM +0800, Walter Wu wrote:
> > > > > > __arm_v7s_unmap() call iopte_deref() to translate pyh_to_virt address,
> > > > > > but it will modify pointer tag into 0xff, so there is a false positive.
> > > > > >
> > > > > > When enable tag-based kasan, phys_to_virt() function need to rewrite
> > > > > > its original pointer tag in order to avoid kasan report an incorrect
> > > > > > memory corruption.
> > > > >
> > > > > Hmm. Which tree did you see this on? We've recently queued a load of fixes
> > > > > in this area, but I /thought/ they were only needed after the support for
> > > > > 52-bit virtual addressing in the kernel.
> > > >
> > > > I'm seeing similar issues in the virtio blk code (splat below), atop of
> > > > the arm64 for-next/core branch. I think this is a latent issue, and
> > > > people are only just starting to test with KASAN_SW_TAGS.
> > > >
> > > > It looks like the virtio blk code will round-trip a SLUB-allocated pointer from
> > > > virt->page->virt, losing the per-object tag in the process.
> > > >
> > > > Our page_to_virt() seems to get a per-page tag, but this only makes
> > > > sense if you're dealing with the page allocator, rather than something
> > > > like SLUB which carves a page into smaller objects giving each object a
> > > > distinct tag.
> > > >
> > > > Any round-trip of a pointer from SLUB is going to lose the per-object
> > > > tag.
> > >
> > > Urgh, I wonder how this is supposed to work?
> > >
> > > If we end up having to check the KASAN shadow for *_to_virt(), then why
> > > do we need to store anything in the page flags at all? Andrey?
> >
> > As per 2813b9c0 ("kasan, mm, arm64: tag non slab memory allocated via
> > pagealloc") we should only save a non-0xff tag in page flags for non
> > slab pages.
> >
> > Could you share your .config so I can reproduce this?
>
> I wrote a test (below) to do so. :)
>
> It fires with arm64 defconfig, + CONFIG_TEST_KASAN=m.
>
> With Andrey Ryabinin's patch it works as expected with no KASAN splats
> for the two new test cases.

OK, Andrey's patch makes sense and fixes both Mark's test patch and
reports from CONFIG_IOMMU_IO_PGTABLE_ARMV7S_SELFTEST.

Tested-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>

on both patches.

>
> Thanks,
> Mark.
>
> ---->8----
> From 7e8569b558fca21ad4e80fddae659591bc84ce1f Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@arm.com>
> Date: Mon, 19 Aug 2019 15:39:32 +0100
> Subject: [PATCH] lib/test_kasan: add roundtrip tests
>
> In several places we needs to be able to operate on pointers which have

"needs" => "need"

> gone via a roundtrip:
>
>         virt -> {phys,page} -> virt
>
> With KASAN_SW_TAGS, we can't preserve the tag for SLUB objects, and the
> {phys,page} -> virt conversion will use KASAN_TAG_KERNEL.
>
> This patch adds tests to ensure that this works as expected, without
> false positives.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Will Deacon <will.deacon@arm.com>
> ---
>  lib/test_kasan.c | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index b63b367a94e8..cf7b93f0d90c 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -19,6 +19,8 @@
>  #include <linux/string.h>
>  #include <linux/uaccess.h>
>
> +#include <asm/page.h>
> +
>  /*
>   * Note: test functions are marked noinline so that their names appear in
>   * reports.
> @@ -337,6 +339,42 @@ static noinline void __init kmalloc_uaf2(void)
>         kfree(ptr2);
>  }
>
> +static noinline void __init kfree_via_page(void)
> +{
> +       char *ptr;
> +       size_t size = 8;
> +       struct page *page;
> +       unsigned long offset;
> +
> +       pr_info("invalid-free false positive (via page)\n");
> +       ptr = kmalloc(size, GFP_KERNEL);
> +       if (!ptr) {
> +               pr_err("Allocation failed\n");
> +               return;
> +       }
> +
> +       page = virt_to_page(ptr);
> +       offset = offset_in_page(ptr);
> +       kfree(page_address(page) + offset);
> +}
> +
> +static noinline void __init kfree_via_phys(void)
> +{
> +       char *ptr;
> +       size_t size = 8;
> +       phys_addr_t phys;
> +
> +       pr_info("invalid-free false positive (via phys)\n");
> +       ptr = kmalloc(size, GFP_KERNEL);
> +       if (!ptr) {
> +               pr_err("Allocation failed\n");
> +               return;
> +       }
> +
> +       phys = virt_to_phys(ptr);
> +       kfree(phys_to_virt(phys));
> +}
> +
>  static noinline void __init kmem_cache_oob(void)
>  {
>         char *p;
> @@ -737,6 +775,8 @@ static int __init kmalloc_tests_init(void)
>         kmalloc_uaf();
>         kmalloc_uaf_memset();
>         kmalloc_uaf2();
> +       kfree_via_page();
> +       kfree_via_phys();
>         kmem_cache_oob();
>         memcg_accounted_kmem_cache();
>         kasan_stack_oob();
> --
> 2.11.0
>

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