Commit d22cc7f6 authored by Qian Cai's avatar Qian Cai Committed by Ingo Molnar

locking/percpu-rwsem: Fix a task_struct refcount

The following commit:

  7f26482a ("locking/percpu-rwsem: Remove the embedded rwsem")

introduced task_struct memory leaks due to messing up the task_struct
refcount.

At the beginning of percpu_rwsem_wake_function(), it calls get_task_struct(),
but if the trylock failed, it will remain in the waitqueue. However, it
will run percpu_rwsem_wake_function() again with get_task_struct() to
increase the refcount but then only call put_task_struct() once the trylock
succeeded.

Fix it by adjusting percpu_rwsem_wake_function() a bit to guard against
when percpu_rwsem_wait() observing !private, terminating the wait and
doing a quick exit() while percpu_rwsem_wake_function() then doing
wake_up_process(p) as a use-after-free.

Fixes: 7f26482a ("locking/percpu-rwsem: Remove the embedded rwsem")
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarQian Cai <cai@lca.pw>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200330213002.2374-1-cai@lca.pw
parent f5e94d10
...@@ -118,14 +118,15 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, ...@@ -118,14 +118,15 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
unsigned int mode, int wake_flags, unsigned int mode, int wake_flags,
void *key) void *key)
{ {
struct task_struct *p = get_task_struct(wq_entry->private);
bool reader = wq_entry->flags & WQ_FLAG_CUSTOM; bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
struct percpu_rw_semaphore *sem = key; struct percpu_rw_semaphore *sem = key;
struct task_struct *p;
/* concurrent against percpu_down_write(), can get stolen */ /* concurrent against percpu_down_write(), can get stolen */
if (!__percpu_rwsem_trylock(sem, reader)) if (!__percpu_rwsem_trylock(sem, reader))
return 1; return 1;
p = get_task_struct(wq_entry->private);
list_del_init(&wq_entry->entry); list_del_init(&wq_entry->entry);
smp_store_release(&wq_entry->private, NULL); smp_store_release(&wq_entry->private, NULL);
......
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