linux-kernel - Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

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Message-ID: <CA+fCnZdj3_+XPtuq15wbdgLxRqXX+ja6vnPCOx3nfR=Z6Q3ChA@mail.gmail.com>
Date: Fri, 7 Mar 2025 02:10:12 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

On Tue, Mar 4, 2025 at 1:31 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.com> wrote:
>
> One other question that came to me about how KASAN works, is there some
> mechanism to prevent data races between two threads? In the compiler perhaps?
>
> For example memory is de-allocated and shadow memory is poisoned but some other
> thread was just about to do a shadow memory check and was interrupted?
>
> I've read the kasan/vmalloc.c comments and from them I'd extrapolate that the
> caller needs to make sure there are not data races / memory barriers are in
> place.

KASAN does nothing to deliberately prevent or detect races. Even if
the race leads to an OOB or UAF, KASAN might not be able to detect it.
But sometimes it does: if poisoned shadow memory values become visible
to the other thread/CPU before it makes a shadow memory value check.