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. 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/[email protected]/ > [2] https://lore.kernel.org/all/[email protected]/ > [3] https://lore.kernel.org/all/[email protected]/ > > 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 >
