linux-kernel - bcachefs: suspicious mm pointer in struct dio

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Message-ID: <CAG48ez21ZtMJ6gcUND6bLV6XD6b--CXmKSRjKq+D33jhRh1LPw@mail.gmail.com>
Date: Wed, 27 Nov 2024 17:57:03 +0100
From: Jann Horn <jannh@...gle.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>, linux-bcachefs@...r.kernel.org
Cc: kernel list <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>, 
	Pavel Begunkov <asml.silence@...il.com>, io-uring <io-uring@...r.kernel.org>
Subject: bcachefs: suspicious mm pointer in struct dio_write

Hi!

In fs/bcachefs/fs-io-direct.c, "struct dio_write" contains a pointer
to an mm_struct. This pointer is grabbed in bch2_direct_write()
(without any kind of refcount increment), and used in
bch2_dio_write_continue() for kthread_use_mm()/kthread_unuse_mm()
which are used to enable userspace memory access from kthread context.
I believe kthread_use_mm()/kthread_unuse_mm() require that the caller
guarantees that the MM hasn't gone through exit_mmap() yet (normally
by holding an mmget() reference).

If we reach this codepath via io_uring, do we have a guarantee that
the mm_struct that called bch2_direct_write() is still alive and
hasn't yet gone through exit_mmap() when it is accessed from
bch2_dio_write_continue()?

I don't know the async direct I/O codepath particularly well, so I
cc'ed the uring maintainers, who probably know this better than me.