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SubjectRe: [PATCH RFC v2] Randomized slab caches for kmalloc()
[Resending this email after noticing I did not reply-to-all]

On Fri, May 12, 2023 at 7:11 PM Gong Ruiqi <gongruiqi1@huawei.com> wrote:
>
>
>
> On 2023/05/11 2:43, Hyeonggon Yoo wrote:
> > I dont think adding a hardening feature by sacrificing one digit
> > percent performance
> > (and additional complexity) is worth. Heap spraying can only occur
> > when the kernel contains
> > security vulnerabilities, and if there is no known ways of performing
> > such an attack,
> > then we would simply be paying a consistent cost.
> >
> > Any opinions from hardening folks?
>
> I did a more throughout performance test on the same machine in the same
> way, and here are the results:
>
> sched/ sched/ syscall/ mem/ mem/
> messaging pipe basic memcpy memset
> control1 0.019 5.459 0.733 15.258789 51.398026
> control2 0.019 5.439 0.730 16.009221 48.828125
> control3 0.019 5.282 0.735 16.009221 48.828125
> control_avg 0.019 5.393 0.733 15.759077 49.684759
>
> exp1 0.019 5.374 0.741 15.500992 46.502976
> exp2 0.019 5.440 0.746 16.276042 51.398026
> exp3 0.019 5.242 0.752 15.258789 51.398026
> exp_avg 0.019 5.352 0.746 15.678608 49.766343
>
> I believe the results show only minor differences and normal
> fluctuation, and no substantial performance degradation.
>
> As Pedro points out in his reply, unfortunately there are always
> security vulnerabilities in the kernel, which is a fact that we have to
> admit. Having a useful mitigation mechanism at the expense of a little
> performance loss would be, in my opinion, quite a good deal in many
> circumstances. And people can still choose not to have it by setting the
> config to n.

Okay, now I don't think I need to tackle it from a performance
perspective anymore, at least it looks like a good tradeoff.

I had few design level concerns (i.e. in ARM64 instructions are 4-byte
aligned) before switching to hash_64(^ random sequence), but looks
good to me now.

> >> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
> >> +# define SLAB_RANDOMSLAB ((slab_flags_t __force)0x01000000U)
> >> +#else
> >> +# define SLAB_RANDOMSLAB 0
> >> +#endif

There is already the SLAB_KMALLOC flag that indicates if a cache is a
kmalloc cache. I think that would be enough for preventing merging
kmalloc caches?

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