Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 15.0 \(3693.60.0.1.1\))
Subject: Re: [RFC PATCH 04/13] vfio/mdev: remove the usage of the list
 iterator after the loop
From:   Jakob <jakobkoschel@gmail.com>
In-Reply-To: <20220218151216.GE1037534@ziepe.ca>
Date:   Wed, 23 Feb 2022 15:18:50 +0100
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnd Bergman <arnd@arndb.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Mike Rapoport <rppt@kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>
Content-Transfer-Encoding: 7bit
Message-Id: <6BA40980-554F-45E2-914D-5E4CD02FF21C@gmail.com>
References: <20220217184829.1991035-1-jakobkoschel@gmail.com>
 <20220217184829.1991035-5-jakobkoschel@gmail.com>
 <20220218151216.GE1037534@ziepe.ca>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Precedence: bulk


> On 18. Feb 2022, at 16:12, Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 
> On Thu, Feb 17, 2022 at 07:48:20PM +0100, Jakob Koschel wrote:
>> It is unsafe to assume that tmp != mdev can only evaluate to false
>> if the break within the list iterator is hit.
>> 
>> When the break is not hit, tmp is set to an address derived from the
>> head element. If mdev would match with that value of tmp it would allow
>> continuing beyond the safety check even if mdev was never found within
>> the list
> 
> I think due to other construction this is not actually possible, but I
> guess it is technically correct
> 
> This seems like just a straight up style fix with nothing to do with
> speculative execution though. Why not just send it as a proper patch?
> 
> Jason

Thank you for your feedback.

I've raised some confusion here, I'm sorry about that.
The idea was to change list_for_each_entry() to set 'pos' to NULL
when the list terminates to avoid invalid usage in speculation.

This will break this code and I therefore included the suggested change
in this RFC. This RFC was not intended to be merged as is.

However, in this example, 'tmp' will be a out-of-bounds pointer
if the loop did finish without hitting the break, so the check past the
loop *could* match 'mdev' even though no break was ever met.

I've now realized that this is probably not realistic iff mdev always
points to a valid struct mdev_device.
(It's a slightly different scenario on PATCH 03/13).

Jakob