Messages in this thread Patch in this message | | | Date | Mon, 19 Aug 2019 16:03:41 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on tag-based kasan |
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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.
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 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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