linux-kernel - Re: [PATCH v2] staging: pi433: add mutex fixing concurrency issues.

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Message-ID: <20180609154842.GB1826@hle-laptop.local>
Date:   Sat, 9 Jun 2018 11:48:42 -0400
From:   Hugo Lefeuvre <hle@....eu.com>
To:     Valentin Vidic <Valentin.Vidic@...Net.hr>
Cc:     devel@...verdev.osuosl.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, kernelnewbies@...nelnewbies.org
Subject: Re: [PATCH v2] staging: pi433: add mutex fixing concurrency issues.

> > After discussing this issue on the kernel newbies mailing list[0] we
> > came to the conclusion that it is very unlikely that pi433_release and
> > pi433_ioctl would ever run concurrently in this case. This is also
> > true for read/write. Unless one can find a situation where this might
> > happen, I think we should not add this potentially unnecessary lock.
> 
> Yes, so we should than drop the TODO comment on this issue?

Well, after taking a closer look it appears that I misunderstood the
TODO.

What this TODO means is that we might run into a whole world of issues
if the device is _removed_ while we're running ioctl or I guess pretty
much any function that accesses the struct pi433_device.

So the issue doesn't come from pi433_release and pi433_ioctl running
concurrently, but rather pi433_ioctl and pi433_remove. Whether this
situation is likely to happen or not is another question which I am
currently taking a look at.

Also, during my work on this driver I found what I'd consider as another
issue: In pi433_ioctl we execute

case PI433_IOC_WR_TX_CFG:
        if (copy_from_user(&instance->tx_cfg, argp,
                   sizeof(struct pi433_tx_cfg)))
            return -EFAULT;
        break;

without any synchronization. What if two ioctl syscalls are called with
command PI433_IOC_WR_TX_CFG ? instance->tx_cfg might get corrupt unless
copy_from_user provides some kind of synchronization, right ?

I have prepared a patch but couldn't test it because I don't have test
devices.

-- 
             Hugo Lefeuvre (hle)    |    www.owl.eu.com
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