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From:   Jann Horn <jannh@google.com>
Date:   Tue, 6 Oct 2020 02:48:59 +0200
Message-ID: <CAG48ez3VKw=B14r-BeAOxGtPExc-G4FYNymRPgFKUKUMsn5Osw@mail.gmail.com>
Subject: Re: [PATCH RFC v2 0/6] Break heap spraying needed for exploiting use-after-free
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Alexander Popov <alex.popov@linux.com>,
        Kees Cook <keescook@chromium.org>,
        Will Deacon <will@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
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On Tue, Oct 6, 2020 at 2:44 AM Matthew Wilcox <willy@infradead.org> wrote:
> On Tue, Oct 06, 2020 at 12:56:33AM +0200, Jann Horn wrote:
> > It seems to me like, if you want to make UAF exploitation harder at
> > the heap allocator layer, you could do somewhat more effective things
> > with a probably much smaller performance budget. Things like
> > preventing the reallocation of virtual kernel addresses with different
> > types, such that an attacker can only replace a UAF object with
> > another object of the same type. (That is not an idea I like very much
> > either, but I would like it more than this proposal.) (E.g. some
> > browsers implement things along those lines, I believe.)
>
> The slab allocator already has that functionality.  We call it
> TYPESAFE_BY_RCU, but if forcing that on by default would enhance security
> by a measurable amount, it wouldn't be a terribly hard sell ...

TYPESAFE_BY_RCU just forces an RCU grace period before the
reallocation; I'm thinking of something more drastic, like completely
refusing to give back the memory, or using vmalloc for slabs where
that's safe (reusing physical but not virtual addresses across types).
And, to make it more effective, something like a compiler plugin to
isolate kmalloc(sizeof(<type>)) allocations by type beyond just size
classes.