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Message-ID: <d3573f72-e476-43ba-8929-c3b6931d06f2@arm.com> Date: Mon, 19 Jan 2026 12:59:57 +0000 From: Ryan Roberts <ryan.roberts@....com> To: Mark Rutland <mark.rutland@....com>, Kees Cook <kees@...nel.org> Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev <agordeev@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, Arnd Bergmann <arnd@...db.de>, "Jason A. Donenfeld" <Jason@...c4.com>, Ard Biesheuvel <ardb@...nel.org>, Jeremy Linton <jeremy.linton@....com>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation On 19/01/2026 10:52, Mark Rutland wrote: > On Fri, Jan 02, 2026 at 01:11:51PM +0000, Ryan Roberts wrote: >> Hi All, > > Hi Ryan, > >> As I reported at [1], kstack offset randomisation suffers from a couple of bugs >> and, on arm64 at least, the performance is poor. This series attempts to fix >> both; patch 1 provides back-portable fixes for the functional bugs. Patches 2-3 >> propose a performance improvement approach. >> >> I've looked at a few different options but ultimately decided that Jeremy's >> original prng approach is the fastest. I made the argument that this approach is >> secure "enough" in the RFC [2] and the responses indicated agreement. > > FWIW, the series all looks good to me. I understand you're likely to > spin a v4 with a couple of minor tweaks (fixing typos and adding an > out-of-line wrapper for a prandom function), but I don't think there's > anything material that needs to change. Thanks for the review, Mark! v4 incomming... > > I've given my Ack on all three patches. I've given the series a quick > boot test (atop v6.19-rc4) with a bunch of debug options enabled, and > all looks well. > > Kees, do you have any comments? It would be nice if we could queue this > up soon. > > Mark. > >> More details in the commit logs. >> >> >> Performance >> =========== >> >> Mean and tail performance of 3 "small" syscalls was measured. syscall was made >> 10 million times and each individually measured and binned. These results have >> low noise so I'm confident that they are trustworthy. >> >> The baseline is v6.18-rc5 with stack randomization turned *off*. So I'm showing >> performance cost of turning it on without any changes to the implementation, >> then the reduced performance cost of turning it on with my changes applied. >> >> **NOTE**: The below results were generated using the RFC patches but there is no >> meaningful change, so the numbers are still valid. >> >> arm64 (AWS Graviton3): >> +-----------------+--------------+-------------+---------------+ >> | Benchmark | Result Class | v6.18-rc5 | per-task-prng | >> | | | rndstack-on | | >> | | | | | >> +=================+==============+=============+===============+ >> | syscall/getpid | mean (ns) | (R) 15.62% | (R) 3.43% | >> | | p99 (ns) | (R) 155.01% | (R) 3.20% | >> | | p99.9 (ns) | (R) 156.71% | (R) 2.93% | >> +-----------------+--------------+-------------+---------------+ >> | syscall/getppid | mean (ns) | (R) 14.09% | (R) 2.12% | >> | | p99 (ns) | (R) 152.81% | 1.55% | >> | | p99.9 (ns) | (R) 153.67% | 1.77% | >> +-----------------+--------------+-------------+---------------+ >> | syscall/invalid | mean (ns) | (R) 13.89% | (R) 3.32% | >> | | p99 (ns) | (R) 165.82% | (R) 3.51% | >> | | p99.9 (ns) | (R) 168.83% | (R) 3.77% | >> +-----------------+--------------+-------------+---------------+ >> >> Because arm64 was previously using get_random_u16(), it was expensive when it >> didn't have any buffered bits and had to call into the crng. That's what caused >> the enormous tail latency. >> >> >> x86 (AWS Sapphire Rapids): >> +-----------------+--------------+-------------+---------------+ >> | Benchmark | Result Class | v6.18-rc5 | per-task-prng | >> | | | rndstack-on | | >> | | | | | >> +=================+==============+=============+===============+ >> | syscall/getpid | mean (ns) | (R) 13.32% | (R) 4.60% | >> | | p99 (ns) | (R) 13.38% | (R) 18.08% | >> | | p99.9 (ns) | 16.26% | (R) 19.38% | >> +-----------------+--------------+-------------+---------------+ >> | syscall/getppid | mean (ns) | (R) 11.96% | (R) 5.26% | >> | | p99 (ns) | (R) 11.83% | (R) 8.35% | >> | | p99.9 (ns) | (R) 11.42% | (R) 22.37% | >> +-----------------+--------------+-------------+---------------+ >> | syscall/invalid | mean (ns) | (R) 10.58% | (R) 2.91% | >> | | p99 (ns) | (R) 10.51% | (R) 4.36% | >> | | p99.9 (ns) | (R) 10.35% | (R) 21.97% | >> +-----------------+--------------+-------------+---------------+ >> >> I was surprised to see that the baseline cost on x86 is 10-12% since it is just >> using rdtsc. But as I say, I believe the results are accurate. >> >> >> Changes since v2 (RFC) [3] >> ========================== >> >> - Moved late_initcall() to initialize kstack_rnd_state out of >> randomize_kstack.h and into main.c. (issue noticed by kernel test robot) >> >> Changes since v1 (RFC) [2] >> ========================== >> >> - Introduced patch 2 to make prandom_u32_state() __always_inline (needed since >> its called from noinstr code) >> - In patch 3, prng is now per-cpu instead of per-task (per Ard) >> >> >> [1] https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/ >> [2] https://lore.kernel.org/all/20251127105958.2427758-1-ryan.roberts@arm.com/ >> [3] https://lore.kernel.org/all/20251215163520.1144179-1-ryan.roberts@arm.com/ >> >> Thanks, >> Ryan >> >> >> Ryan Roberts (3): >> randomize_kstack: Maintain kstack_offset per task >> prandom: Convert prandom_u32_state() to __always_inline >> randomize_kstack: Unify random source across arches >> >> arch/Kconfig | 5 ++- >> arch/arm64/kernel/syscall.c | 11 ------ >> arch/loongarch/kernel/syscall.c | 11 ------ >> arch/powerpc/kernel/syscall.c | 12 ------- >> arch/riscv/kernel/traps.c | 12 ------- >> arch/s390/include/asm/entry-common.h | 8 ----- >> arch/x86/include/asm/entry-common.h | 12 ------- >> include/linux/prandom.h | 19 +++++++++- >> include/linux/randomize_kstack.h | 54 +++++++++++----------------- >> init/main.c | 9 ++++- >> kernel/fork.c | 1 + >> lib/random32.c | 19 ---------- >> 12 files changed, 49 insertions(+), 124 deletions(-) >> >> -- >> 2.43.0 >>
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