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Donenfeld" To: Ryan Roberts Cc: Arnd Bergmann , Kees Cook , Ard Biesheuvel , Jeremy Linton , Will Deacon , Catalin Marinas , Mark Rutland , "linux-arm-kernel@lists.infradead.org" , Linux Kernel Mailing List , ebiggers@kernel.org Subject: Re: [DISCUSSION] kstack offset randomization: bugs and performance Message-ID: References: <66c4e2a0-c7fb-46c2-acce-8a040a71cd8e@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Tue, Nov 18, 2025 at 05:21:17PM +0000, Ryan Roberts wrote: > On 18/11/2025 17:15, Jason A. Donenfeld wrote: > > On Mon, Nov 17, 2025 at 05:47:05PM +0100, Arnd Bergmann wrote: > >> On Mon, Nov 17, 2025, at 12:31, Ryan Roberts wrote: > >>> On 17/11/2025 11:30, Ryan Roberts wrote: > >>>> Hi All, > >>>> > >>>> Over the last few years we had a few complaints that syscall performance on > >>>> arm64 is slower than x86. Most recently, it was observed that a certain Java > >>>> benchmark that does a lot of fstat and lseek is spending ~10% of it's time in > >>>> get_random_u16(). Cue a bit of digging, which led me to [1] and also to some new > >>>> ideas about how performance could be improved. > >> > >> > >>>> I believe this helps the mean latency significantly without sacrificing any > >>>> strength. But it doesn't reduce the tail latency because we still have to call > >>>> into the crng eventually. > >>>> > >>>> So here's another idea: Could we use siphash to generate some random bits? We > >>>> would generate the secret key at boot using the crng. Then generate a 64 bit > >>>> siphash of (cntvct_el0 ^ tweak) (where tweak increments every time we generate a > >>>> new hash). As long as the key remains secret, the hash is unpredictable. > >>>> (perhaps we don't even need the timer value). For every hash we get 64 bits, so > >>>> that would last for 10 syscalls at 6 bits per call. So we would still have to > >>>> call siphash every 10 syscalls, so there would still be a tail, but from my > >>>> experiements, it's much less than the crng: > >> > >> IIRC, Jason argued against creating another type of prng inside of the > >> kernel for a special purpose. > > > > Yes indeed... I'm really not a fan of adding bespoke crypto willynilly > > like that. Let's make get_random_u*() faster. If you're finding that the > > issue with it is the locking, and that you're calling this from irq > > context anyway, then your proposal (if I read this discussion correctly) > > to add a raw_get_random_u*() seems like it could be sensible. Those > > functions are generated via macro anyway, so it wouldn't be too much to > > add the raw overloads. Feel free to send a patch to my random.git tree > > if you'd like to give that a try. > > Thanks Jason; that's exactly what I did, and it helps. But I think ultimately > the get_random_uXX() slow path is too slow; that's the part that causes the tail > latency problem. I doubt there are options for speeding that up? > > Anyway, I'm currently prototyping a few options and getting clear performance > numbers. I'll be back in a couple of days and we can continue the discussion in > light of the data. Interesting... I would be curious to see what sorts of stable numbers you find. Because most of the time, get_random_uXX() should just be copying memory. Does the unlikely slower case really matter that much? I suspect it doesn't matter for anything real. On the other hand, it's probably possible to improve the slow path on ARM a bit by using the pure-ARM assembly chacha implementation that we use in the vDSO: https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/tree/arch/arm64/kernel/vdso/vgetrandom-chacha.S Or by using the non-generic code already provided by libcrypto from random.c. Jason