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

[Walter Wu]

On Fri, 2019-06-14 at 10:32 +0800, Walter Wu wrote:
> On Fri, 2019-06-14 at 01:46 +0800, Walter Wu wrote:
> > 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);
> > }
> > ---
> > 
> We think of another question about deciding by that shadow of the first
> byte.
> In tag-based KASAN, it is immediately released after calling kfree(), so
> the slub is easy to be used by another pointer, then it will change
> shadow memory to the tag of new pointer, it will not be the
> KASAN_TAG_INVALID, so there are many false negative cases, especially in
> small size allocation.
> 
> Our patch is to solve those problems. so please consider it, thanks.
> 
Hi, Andrey and Dmitry,

I am sorry to bother you.
Would you tell me what you think about this patch?
We want to use tag-based KASAN, so we hope its bug report is clear and
correct as generic KASAN.

Thanks your review.
Walter