Messages in this thread Patch in this message | | | Date | Tue, 17 Nov 2020 17:36:41 +0100 | Subject | [PATCH] kcsan: Avoid scheduler recursion by using non-instrumented preempt_disable() | From | Marco Elver <> |
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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() 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> --- kernel/kcsan/core.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c index 3994a217bde7..967b0b2f9d59 100644 --- a/kernel/kcsan/core.c +++ b/kernel/kcsan/core.c @@ -284,10 +284,18 @@ 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 version of + * preempt_disable() 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(); - put_cpu_var(kcsan_rand_state); return (u32)(((u64) res * ep_ro) >> 32); } -- 2.29.2.299.gdc1121823c-goog
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