6.6-stable review patch. If anyone has any objections, please let me know.
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From: Linus Torvalds torvalds@linux-foundation.org
commit 8c2e52ebbe885c7eeaabd3b7ddcdc1246fc400d2 upstream.
Jann Horn points out that epoll is decrementing the ep refcount and then doing a
mutex_unlock(&ep->mtx);
afterwards. That's very wrong, because it can lead to a use-after-free.
That pattern is actually fine for the very last reference, because the code in question will delay the actual call to "ep_free(ep)" until after it has unlocked the mutex.
But it's wrong for the much subtler "next to last" case when somebody *else* may also be dropping their reference and free the ep while we're still using the mutex.
Note that this is true even if that other user is also using the same ep mutex: mutexes, unlike spinlocks, can not be used for object ownership, even if they guarantee mutual exclusion.
A mutex "unlock" operation is not atomic, and as one user is still accessing the mutex as part of unlocking it, another user can come in and get the now released mutex and free the data structure while the first user is still cleaning up.
See our mutex documentation in Documentation/locking/mutex-design.rst, in particular the section [1] about semantics:
"mutex_unlock() may access the mutex structure even after it has internally released the lock already - so it's not safe for another context to acquire the mutex and assume that the mutex_unlock() context is not using the structure anymore"
So if we drop our ep ref before the mutex unlock, but we weren't the last one, we may then unlock the mutex, another user comes in, drops _their_ reference and releases the 'ep' as it now has no users - all while the mutex_unlock() is still accessing it.
Fix this by simply moving the ep refcount dropping to outside the mutex: the refcount itself is atomic, and doesn't need mutex protection (that's the whole _point_ of refcounts: unlike mutexes, they are inherently about object lifetimes).
Reported-by: Jann Horn jannh@google.com Link: https://docs.kernel.org/locking/mutex-design.html#semantics [1] Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Christian Brauner brauner@kernel.org Cc: Jan Kara jack@suse.cz Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/eventpoll.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-)
--- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -772,7 +772,7 @@ static bool __ep_remove(struct eventpoll call_rcu(&epi->rcu, epi_rcu_free);
percpu_counter_dec(&ep->user->epoll_watches); - return ep_refcount_dec_and_test(ep); + return true; }
/* @@ -780,14 +780,14 @@ static bool __ep_remove(struct eventpoll */ static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi) { - WARN_ON_ONCE(__ep_remove(ep, epi, false)); + if (__ep_remove(ep, epi, false)) + WARN_ON_ONCE(ep_refcount_dec_and_test(ep)); }
static void ep_clear_and_put(struct eventpoll *ep) { struct rb_node *rbp, *next; struct epitem *epi; - bool dispose;
/* We need to release all tasks waiting for these file */ if (waitqueue_active(&ep->poll_wait)) @@ -820,10 +820,8 @@ static void ep_clear_and_put(struct even cond_resched(); }
- dispose = ep_refcount_dec_and_test(ep); mutex_unlock(&ep->mtx); - - if (dispose) + if (ep_refcount_dec_and_test(ep)) ep_free(ep); }
@@ -1003,7 +1001,7 @@ again: dispose = __ep_remove(ep, epi, true); mutex_unlock(&ep->mtx);
- if (dispose) + if (dispose && ep_refcount_dec_and_test(ep)) ep_free(ep); goto again; }