Re: [PATCH v3] kasan: add memory corruption identification for software tag-based mode

[Walter Wu]

On Thu, 2019-06-13 at 15:27 +0300, Andrey Ryabinin wrote:
> 
> On 6/13/19 11:13 AM, Walter Wu wrote:
> > This patch adds memory corruption identification at bug report for
> > software tag-based mode, the report show whether it is "use-after-free"
> > or "out-of-bound" error instead of "invalid-access" error.This will make
> > it easier for programmers to see the memory corruption problem.
> > 
> > Now we extend the quarantine to support both generic and tag-based kasan.
> > For tag-based kasan, the quarantine stores only freed object information
> > to check if an object is freed recently. When tag-based kasan reports an
> > error, we can check if the tagged addr is in the quarantine and make a
> > good guess if the object is more like "use-after-free" or "out-of-bound".
> > 
> 
> 
> We already have all the information and don't need the quarantine to make such guess.
> Basically if shadow of the first byte of object has the same tag as tag in pointer than it's out-of-bounds,
> otherwise it's use-after-free.
> 
> In pseudo-code it's something like this:
> 
> u8 object_tag = *(u8 *)kasan_mem_to_shadow(nearest_object(cacche, page, access_addr));
> 
> if (access_addr_tag == object_tag && object_tag != KASAN_TAG_INVALID)
> 	// out-of-bounds
> else
> 	// use-after-free

Thanks your explanation.
I see, we can use it to decide corruption type.
But some use-after-free issues, it may not have accurate free-backtrace.
Unfortunately in that situation, free-backtrace is the most important.
please see below example

In generic KASAN, it gets accurate free-backrace(ptr1).
In tag-based KASAN, it gets wrong free-backtrace(ptr2). It will make
programmer misjudge, so they may not believe tag-based KASAN.
So We provide this patch, we hope tag-based KASAN bug report is the same
accurate with generic KASAN.

---
    ptr1 = kmalloc(size, GFP_KERNEL);
    ptr1_free(ptr1);

    ptr2 = kmalloc(size, GFP_KERNEL);
    ptr2_free(ptr2);

    ptr1[size] = 'x';  //corruption here


static noinline void ptr1_free(char* ptr)
{
    kfree(ptr);
}
static noinline void ptr2_free(char* ptr)
{
    kfree(ptr);
}
---