linux-kernel - Re: [RFC PATCH 04/13] vfio/mdev: remove the usage of the list iterator after the loop

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Message-Id: <6BA40980-554F-45E2-914D-5E4CD02FF21C@gmail.com>
Date:   Wed, 23 Feb 2022 15:18:50 +0100
From:   Jakob <jakobkoschel@...il.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergman <arnd@...db.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [RFC PATCH 04/13] vfio/mdev: remove the usage of the list
 iterator after the loop



> On 18. Feb 2022, at 16:12, Jason Gunthorpe <jgg@...pe.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