From: David Laight <David.Laight@ACULAB.COM>
To: 'Jann Horn' <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
"Pavel Begunkov" <asml.silence@gmail.com>,
io-uring <io-uring@vger.kernel.org>
Cc: kernel list <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Waiman Long <longman@redhat.com>
Subject: RE: io_uring: incorrect assumption about mutex behavior on unlock?
Date: Fri, 1 Dec 2023 18:30:05 +0000 [thread overview]
Message-ID: <811a97651e144b83a35fd7eb713aeeae@AcuMS.aculab.com> (raw)
In-Reply-To: <CAG48ez3xSoYb+45f1RLtktROJrpiDQ1otNvdR+YLQf7m+Krj5Q@mail.gmail.com>
From: Jann Horn
> Sent: 01 December 2023 16:41
>
> mutex_unlock() has a different API contract compared to spin_unlock().
> spin_unlock() can be used to release ownership of an object, so that
> as soon as the spinlock is unlocked, another task is allowed to free
> the object containing the spinlock.
> mutex_unlock() does not support this kind of usage: The caller of
> mutex_unlock() must ensure that the mutex stays alive until
> mutex_unlock() has returned.
The problem sequence might be:
Thread A Thread B
mutex_lock()
code to stop mutex being requested
...
mutex_lock() - sleeps
mutex_unlock()...
Waiters woken...
isr and/or pre-empted
- wakes up
mutex_unlock()
free()
... more kernel code access the mutex
BOOOM
What happens in a PREEMPT_RT kernel where most of the spin_unlock()
get replaced by mutex_unlock().
Seems like they can potentially access a freed mutex?
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2023-12-01 18:30 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-01 16:41 Jann Horn
2023-12-01 18:30 ` David Laight [this message]
2023-12-01 18:40 ` mutex/spinlock semantics [was: Re: io_uring: incorrect assumption about mutex behavior on unlock?] Jann Horn
2023-12-01 18:52 ` io_uring: incorrect assumption about mutex behavior on unlock? Pavel Begunkov
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