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

Mark Rutland mark.rutland at arm.com
Mon Aug 19 08:03:41 PDT 2019


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 at 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 at 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 at arm.com>
Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
Cc: Andrey Konovalov <andreyknvl at google.com>
Cc: Will Deacon <will.deacon at 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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