From: Paul E. McKenney paulmck@kernel.org
[ Upstream commit 3066820034b5dd4e89bd74a7739c51c2d6f5e554 ]
If another lockdep report runs concurrently with an RCU lockdep report from RCU_LOCKDEP_WARN(), the following sequence of events can occur:
1. debug_lockdep_rcu_enabled() sees that lockdep is enabled when called from (say) synchronize_rcu().
2. Lockdep is disabled by a concurrent lockdep report.
3. debug_lockdep_rcu_enabled() evaluates its lockdep-expression argument, for example, lock_is_held(&rcu_bh_lock_map).
4. Because lockdep is now disabled, lock_is_held() plays it safe and returns the constant 1.
5. But in this case, the constant 1 is not safe, because invoking synchronize_rcu() under rcu_read_lock_bh() is disallowed.
6. debug_lockdep_rcu_enabled() wrongly invokes lockdep_rcu_suspicious(), resulting in a false-positive splat.
This commit therefore changes RCU_LOCKDEP_WARN() to check debug_lockdep_rcu_enabled() after checking the lockdep expression, so that any "safe" returns from lock_is_held() are rejected by debug_lockdep_rcu_enabled(). This requires memory ordering, which is supplied by READ_ONCE(debug_locks). The resulting volatile accesses prevent the compiler from reordering and the fact that only one variable is being accessed prevents the underlying hardware from reordering. The combination works for IA64, which can reorder reads to the same location, but this is defeated by the volatile accesses, which compile to load instructions that provide ordering.
Reported-by: syzbot+dde0cc33951735441301@syzkaller.appspotmail.com Reported-by: Matthew Wilcox willy@infradead.org Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com Reported-by: Thomas Gleixner tglx@linutronix.de Suggested-by: Boqun Feng boqun.feng@gmail.com Reviewed-by: Boqun Feng boqun.feng@gmail.com Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/rcupdate.h | 2 +- kernel/rcu/update.c | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index c5adba5e79e7..7d12c76e8fa4 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -308,7 +308,7 @@ static inline int rcu_read_lock_any_held(void) #define RCU_LOCKDEP_WARN(c, s) \ do { \ static bool __section(".data.unlikely") __warned; \ - if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ + if ((c) && debug_lockdep_rcu_enabled() && !__warned) { \ __warned = true; \ lockdep_rcu_suspicious(__FILE__, __LINE__, s); \ } \ diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 39334d2d2b37..849f0aa99333 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -275,7 +275,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
noinstr int notrace debug_lockdep_rcu_enabled(void) { - return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && + return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) && current->lockdep_recursion == 0; } EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);