bpf: Allow bpf_spin_{lock,unlock} in sleepable progs

Commit 9e7a4d98 ("bpf: Allow LSM programs to use bpf spin locks")
disabled bpf_spin_lock usage in sleepable progs, stating:

 Sleepable LSM programs can be preempted which means that allowng spin
 locks will need more work (disabling preemption and the verifier
 ensuring that no sleepable helpers are called when a spin lock is
 held).

This patch disables preemption before grabbing bpf_spin_lock. The second
requirement above "no sleepable helpers are called when a spin lock is
held" is implicitly enforced by current verifier logic due to helper
calls in spin_lock CS being disabled except for a few exceptions, none
of which sleep.

Due to above preemption changes, bpf_spin_lock CS can also be considered
a RCU CS, so verifier's in_rcu_cs check is modified to account for this.
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230821193311.3290257-7-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -286,6 +286,7 @@ static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
 	compiletime_assert(u.val == 0, "__ARCH_SPIN_LOCK_UNLOCKED not 0");
 	BUILD_BUG_ON(sizeof(*l) != sizeof(__u32));
 	BUILD_BUG_ON(sizeof(*lock) != sizeof(__u32));
+	preempt_disable();
 	arch_spin_lock(l);
 }
 
@@ -294,6 +295,7 @@ static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
 	arch_spinlock_t *l = (void *)lock;
 
 	arch_spin_unlock(l);
+	preempt_enable();
 }
 
 #else

kernel/bpf/verifier.c

@@ -5064,7 +5064,9 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
  */
 static bool in_rcu_cs(struct bpf_verifier_env *env)
 {
-	return env->cur_state->active_rcu_lock || !env->prog->aux->sleepable;
+	return env->cur_state->active_rcu_lock ||
+	       env->cur_state->active_lock.ptr ||
+	       !env->prog->aux->sleepable;
 }
 
 /* Once GCC supports btf_type_tag the following mechanism will be replaced with tag check */
@@ -16975,11 +16977,6 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
 			verbose(env, "tracing progs cannot use bpf_spin_lock yet\n");
 			return -EINVAL;
 		}
-
-		if (prog->aux->sleepable) {
-			verbose(env, "sleepable progs cannot use bpf_spin_lock yet\n");
-			return -EINVAL;
-		}
 	}
 
 	if (btf_record_has_field(map->record, BPF_TIMER)) {