Re: [RFC PATCH 0/2] kasan: hw_tags: Add option to tag only at allocation time

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jun 18, 2026 at 10:35:15PM +0900, Harry Yoo wrote:
> On 6/12/26 1:44 PM, Dev Jain wrote:
> > Introduce a boot option to tag only at allocation time of the objects. This
> > reduces KASAN MTE overhead, the tradeoff being reduced ability of
> > catching bugs.
> 
> I think most of overhead when enabling MTE comes from loading and
> validing tags for every memory access (either in SYNC or ASYNC mode),
> rather than from storing tags.

I guess it depends on the workload. Lots of allocations for short-lived
buffers (e.g. network traffic) may notice the additional tagging more
than the actual tag checking.

Of course, it would be nice to get some numbers from those who have
access to MTE capable hardware.

> > Now, when a memory object will be freed, it will retain the random tag it
> > had at allocation time. This compromises on catching UAF bugs, till the
> > time the object is not reallocated, at which point it will have a new
> > random tag.
> > 
> > Hence, not catching "use-after-free-before-reallocation" and not catching
> > "double-free" will be the compromise for reduced KASAN overhead.
> 
> I doubt users who care about security enough to enable HW_TAGS KASAN
> are willing to compromise on security just to save a few instructions
> to store tags in the free path.
> 
> To me, it looks like too much of a compromise on security for little
> performance gain.

I don't think there's much compromise on security for use-after-free.
The buffer will be re-tagged later so use-after-realloc should be
caught, especially if we ensure that a different tag will be used (I
don't think Dev's patches do this).

Of course, if you want MTE as a debug/bug-finding feature, tagging on
both allocation and freeing is highly recommended. This patchset is
aimed for those wanting to run MTE in production and squeeze a bit more
performance out of it (with the compromise of not detecting
use-after-free, only prevent access after re-allocation).

-- 
Catalin