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

[Ryan Roberts <ryan.roberts@arm.com>]

On 12/06/2026 05:44, 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.
> 
> 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.

Does standard KASAN with HW_TAGS really detect double-free? How does it do that?
I could imagine it testing the tags of memory being freed to see if they are set
to the poison tag, but that would lead to false positives for the GFP_SKIP_KASAN
case, surely?

If I'm right, then the only downgrade this new mode causes is that if
freed-but-not-yet-reallocated memory is accessed via it's dangling pointer, then
that bad access is not detected. I think that would be benign in all the cases I
can think of, so while it would be a problem for a debugging use case, it would
unlikely be a problem for security enforcement?

Thanks,
Ryan


> 
> This is an RFC because we are not clear about the performance benefit.
> 
> Android folks, please help with testing!
> 
> ---
> Applies on Linus master (9716c086c8e8).
> 
> Dev Jain (2):
>   kasan: hw_tags: Use KASAN_PAGE_REDZONE for vmalloc redzoning
>   kasan: hw_tags: Add boot option to elide free time poisoning
> 
>  Documentation/dev-tools/kasan.rst |  4 +++
>  mm/kasan/hw_tags.c                | 45 +++++++++++++++++++++++++++++--
>  mm/kasan/kasan.h                  | 23 +++++++++++++++-
>  3 files changed, 69 insertions(+), 3 deletions(-)
>