linux-kernel - io_uring vs in_compat

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Message-ID: <20200720061046.GA10678@lst.de>
Date:   Mon, 20 Jul 2020 08:10:46 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: io_uring vs in_compat_syscall()

Hi Jens,

I just found a (so far theoretical) issue with the io_uring submission
offloading to workqueues or threads.  We have lots of places using
in_compat_syscall() to check if a syscall needs compat treatmenet.
While the biggest users is iocttl(), we also have a fair amount of
places using in_compat_task() in read and write methods, and these
will not do the wrong thing when used with io_uring under certain
conditions.  I'm not sure how to best fix this, except for making sure
in_compat_syscall() returns true one way or another for these cases.