From 445dc58add71f976c20359568d370f5059d30f77 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Sat, 20 Jul 2019 10:45:39 -0400 Subject: [PATCH] locking/rwsem: Don't call owner_on_cpu() on read-owner For writer, the owner value is cleared on unlock. For reader, it is left intact on unlock for providing better debugging aid on crash dump and the unlock of one reader may not mean the lock is free. As a result, the owner_on_cpu() shouldn't be used on read-owner as the task pointer value may not be valid and it might have been freed. That is the case in rwsem_spin_on_owner(), but not in rwsem_can_spin_on_owner(). This can lead to use-after-free error from KASAN. For example, BUG: KASAN: use-after-free in rwsem_down_write_slowpath (/home/miguel/kernel/linux/kernel/locking/rwsem.c:669 /home/miguel/kernel/linux/kernel/locking/rwsem.c:1125) Fix this by checking for RWSEM_READER_OWNED flag before calling owner_on_cpu(). Fixes: 94a9717b3c40e ("locking/rwsem: Make rwsem->owner an atomic_long_t") Signed-off-by: Waiman Long --- kernel/locking/rwsem.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 37524a47f002..bc91aacaab58 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -666,7 +666,11 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, preempt_disable(); rcu_read_lock(); owner = rwsem_owner_flags(sem, &flags); - if ((flags & nonspinnable) || (owner && !owner_on_cpu(owner))) + /* + * Don't check the read-owner as the entry may be stale. + */ + if ((flags & nonspinnable) || + (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner))) ret = false; rcu_read_unlock(); preempt_enable(); -- 2.18.1