linux-kernel - Re: [PATCH] fsi:fsi-sbefifo: Fix possible concurrency use-after-free bugs in sbefifo_user

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Message-ID: <a2aae05948a2c58f512e7393822e0768209069e2.camel@kernel.crashing.org>
Date:   Thu, 03 Jan 2019 15:26:28 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     David Howells <dhowells@...hat.com>,
        Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     joel@....id.au, eajames@...ux.vnet.ibm.com, andrew@...id.au,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fsi:fsi-sbefifo: Fix possible concurrency
 use-after-free bugs in sbefifo_user_release

On Thu, 2019-01-03 at 14:27 +1100, Benjamin Herrenschmidt wrote:
> On Wed, 2019-01-02 at 09:34 +0000, David Howells wrote:
> > Jia-Ju Bai <baijiaju1990@...il.com> wrote:
> > 
> > > +	mutex_lock(&user->file_lock);
> > >  	sbefifo_release_command(user);
> > >  	free_page((unsigned long)user->cmd_page);
> > > +	mutex_unlock(&user->file_lock);
> > 
> > It shouldn't be necessary to do the free_page() call inside the locked
> > section.
> 
> True. However, I didn't realize read/write could be concurrent with
> release so we have another problem.
> 
> I assume when release is called, no new read/write can be issued, I am
> correct ? So all we have to protect against is a read/write that has
> started prior to release being called, right ?

Hrm... looking briefly at the vfs, read/write are wrapped in
fdget/fdput, so release shouldn't happen concurrently or am I missing
something here ?

Cheers,
Ben.