Hi Kees, I'm keen to get some testing in linux-next and hopefully get this upstream for v7.1 as we previously discussed. Are you willing/able to take this via your tree?
Thanks, Ryan On 03/03/2026 15:08, Ryan Roberts wrote: > [Kees; I'm hoping this is now good-to-go via your hardening tree? It would be > good to get some linux-next testing.] > > Hi All, > > 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. Patch 2 > proposes 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. > > 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. I've also rerun the tests > with this version on top of v7.0-rc2 on arm64 and confirmed simialr results. > > arm64 (AWS Graviton3): > +-----------------+--------------+-------------+---------------+ > | Benchmark | Result Class | v6.18-rc5 | per-cpu-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-cpu-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 v4 [5] > ==================== > > - Moved add_random_kstack_offset() later in syscall entry code for powerpc, > s390 > and x86. On these platforms it was previously within noinstr sections but > for > some exotic Kconfigs, [get|put]_cpu_var() was calling out to instrumentable > code. (reported by kernel test robot) > - Removed what was previously patch 2 (inline version of prandom_u32_state()). > With the above change, there is no longer an issue with calling the > out-of-line version. > > Changes since v3 [4] > ==================== > > - Patch 1: Fixed typo in commit log (per David L) > - Patch 2: Reinstated prandom_u32_state() as out-of-line function, which > forwards to inline version (per David L) > - Patch 3: Added supplementary info about benefits of removing > choose_random_kstack_offset() (per Mark R) > > Changes since v2 [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]/ > [4] https://lore.kernel.org/all/[email protected]/ > [5] https://lore.kernel.org/all/[email protected] > > Thanks, > Ryan > > > Ryan Roberts (2): > randomize_kstack: Maintain kstack_offset per task > 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 | 16 ++------- > arch/riscv/kernel/traps.c | 12 ------- > arch/s390/include/asm/entry-common.h | 8 ----- > arch/s390/kernel/syscall.c | 2 +- > arch/x86/entry/syscall_32.c | 4 +-- > arch/x86/entry/syscall_64.c | 2 +- > arch/x86/include/asm/entry-common.h | 12 ------- > include/linux/randomize_kstack.h | 54 +++++++++++----------------- > init/main.c | 9 ++++- > kernel/fork.c | 1 + > 13 files changed, 37 insertions(+), 110 deletions(-) > > -- > 2.43.0 >
