From: Kees Cook <keescook@chromium.org>
To: Jeremy Linton <jeremy.linton@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
will@kernel.org, Jason@zx2c4.com, gustavoars@kernel.org,
mark.rutland@arm.com, rostedt@goodmis.org, arnd@arndb.de,
broonie@kernel.org, guohui@uniontech.com, Manoj.Iyer@arm.com,
linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
James Yang <james.yang@arm.com>,
Shiyou Huang <shiyou.huang@arm.com>
Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization
Date: Tue, 5 Mar 2024 15:33:54 -0800 [thread overview]
Message-ID: <202403051526.0BE26F99E@keescook> (raw)
In-Reply-To: <20240305221824.3300322-2-jeremy.linton@arm.com>
On Tue, Mar 05, 2024 at 04:18:24PM -0600, Jeremy Linton wrote:
> The existing arm64 stack randomization uses the kernel rng to acquire
> 5 bits of address space randomization. This is problematic because it
> creates non determinism in the syscall path when the rng needs to be
> generated or reseeded. This shows up as large tail latencies in some
> benchmarks and directly affects the minimum RT latencies as seen by
> cyclictest.
>
> Other architectures are using timers/cycle counters for this function,
> which is sketchy from a randomization perspective because it should be
> possible to estimate this value from knowledge of the syscall return
> time, and from reading the current value of the timer/counters.
>
> So, a poor rng should be better than the cycle counter if it is hard
> to extract the stack offsets sufficiently to be able to detect the
> PRNG's period. Lets downgrade from get_random_u16() to
> prandom_u32_state() under the theory that the danger of someone
> guessing the 1 in 32 per call offset, is larger than that of being
> able to extract sufficient history to accurately predict future
> offsets. Further it should be safer to run with prandom_u32_state than
> disabling stack randomization for those subset of applications where the
> difference in latency is on the order of ~5X worse.
>
> Reported-by: James Yang <james.yang@arm.com>
> Reported-by: Shiyou Huang <shiyou.huang@arm.com>
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
> arch/arm64/kernel/syscall.c | 42 ++++++++++++++++++++++++++++++++++++-
> 1 file changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> index 9a70d9746b66..33b3ea4adff8 100644
> --- a/arch/arm64/kernel/syscall.c
> +++ b/arch/arm64/kernel/syscall.c
> @@ -5,6 +5,7 @@
> #include <linux/errno.h>
> #include <linux/nospec.h>
> #include <linux/ptrace.h>
> +#include <linux/prandom.h>
> #include <linux/randomize_kstack.h>
> #include <linux/syscalls.h>
>
> @@ -37,6 +38,45 @@ static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
> return syscall_fn(regs);
> }
>
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +DEFINE_PER_CPU(struct rnd_state, kstackrng);
> +
> +static u16 kstack_rng(void)
> +{
> + u32 rng = prandom_u32_state(this_cpu_ptr(&kstackrng));
> +
> + return rng & 0x1ff;
> +}
> +
> +/* Should we reseed? */
> +static int kstack_rng_setup(unsigned int cpu)
> +{
> + u32 rng_seed;
> +
> + /* zero should be avoided as a seed */
> + do {
> + rng_seed = get_random_u32();
> + } while (!rng_seed);
> + prandom_seed_state(this_cpu_ptr(&kstackrng), rng_seed);
> + return 0;
> +}
> +
> +static int kstack_init(void)
> +{
> + int ret;
> +
> + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "arm64/cpuinfo:kstackrandomize",
> + kstack_rng_setup, NULL);
This will run initial seeding, but don't we need to reseed this with
some kind of frequency?
Otherwise, seems fine to me.
--
Kees Cook
next prev parent reply other threads:[~2024-03-05 23:33 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-05 22:18 [PATCH 0/1] Bring kstack randomized perf closer to unrandomized Jeremy Linton
2024-03-05 22:18 ` [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization Jeremy Linton
2024-03-05 23:33 ` Kees Cook [this message]
2024-03-06 20:46 ` Arnd Bergmann
2024-03-06 21:54 ` Jeremy Linton
2024-03-07 11:10 ` Arnd Bergmann
2024-03-07 19:10 ` Kees Cook
2024-03-07 21:56 ` Arnd Bergmann
2024-03-07 19:15 ` Kees Cook
2024-03-07 22:02 ` Arnd Bergmann
2024-03-08 16:49 ` Jeremy Linton
2024-03-08 20:29 ` Arnd Bergmann
2024-03-22 23:40 ` Jeremy Linton
2024-03-23 12:47 ` Arnd Bergmann
2024-03-07 19:05 ` kernel test robot
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