Messages in this thread | | | Date | Mon, 23 Nov 2020 14:55:12 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] kcsan: Avoid scheduler recursion by using non-instrumented preempt_{disable,enable}() |
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On Mon, Nov 23, 2020 at 02:23:00PM +0100, Marco Elver wrote: > When enabling KCSAN for kernel/sched (remove KCSAN_SANITIZE := n from > kernel/sched/Makefile), with CONFIG_DEBUG_PREEMPT=y, we can observe > recursion due to: > > check_access() [via instrumentation] > kcsan_setup_watchpoint() > reset_kcsan_skip() > kcsan_prandom_u32_max() > get_cpu_var() > preempt_disable() > preempt_count_add() [in kernel/sched/core.c] > check_access() [via instrumentation] > > Avoid this by rewriting kcsan_prandom_u32_max() to only use safe > versions of preempt_disable() and preempt_enable() that do not call into > scheduler code. > > Note, while this currently does not affect an unmodified kernel, it'd be > good to keep a KCSAN kernel working when KCSAN_SANITIZE := n is removed > from kernel/sched/Makefile to permit testing scheduler code with KCSAN > if desired. > > Fixes: cd290ec24633 ("kcsan: Use tracing-safe version of prandom") > Signed-off-by: Marco Elver <elver@google.com> > --- > v2: > * Update comment to also point out preempt_enable(). > --- > kernel/kcsan/core.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c > index 3994a217bde7..10513f3e2349 100644 > --- a/kernel/kcsan/core.c > +++ b/kernel/kcsan/core.c > @@ -284,10 +284,19 @@ should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx * > */ > static u32 kcsan_prandom_u32_max(u32 ep_ro) > { > - struct rnd_state *state = &get_cpu_var(kcsan_rand_state); > - const u32 res = prandom_u32_state(state); > + struct rnd_state *state; > + u32 res; > + > + /* > + * Avoid recursion with scheduler by using non-tracing versions of > + * preempt_disable() and preempt_enable() that do not call into > + * scheduler code. > + */ > + preempt_disable_notrace(); > + state = raw_cpu_ptr(&kcsan_rand_state); > + res = prandom_u32_state(state); > + preempt_enable_no_resched_notrace();
This is a preemption bug. Does preempt_enable_notrace() not work?
> > - put_cpu_var(kcsan_rand_state); > return (u32)(((u64) res * ep_ro) >> 32); > } > > -- > 2.29.2.454.gaff20da3a2-goog >
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