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

[Mark Rutland]

On Mon, Aug 19, 2019 at 05:37:36PM +0200, Andrey Konovalov wrote:
> On Mon, Aug 19, 2019 at 5:03 PM Mark Rutland <mark.rutland@xxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> on both patches.
> 
> >
> > Thanks,
> > Mark.
> >
> > ---->8----
> > From 7e8569b558fca21ad4e80fddae659591bc84ce1f Mon Sep 17 00:00:00 2001
> > From: Mark Rutland <mark.rutland@xxxxxxx>
> > 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"

Thanks! 

I'll spin a standalone v2 of this with that fixed and your tags folded
in.

Mark.

> 
> > 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@xxxxxxx>
> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > ---
> >  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
> >