Re: [PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation

[Ryan Roberts <ryan.roberts@arm.com>]

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
>>