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

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Date:   Wed, 23 Feb 2022 15:12:22 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jakob <jakobkoschel@...il.com>,
        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 Wed, Feb 23, 2022 at 11:06:03AM -0800, Linus Torvalds wrote:

> And as such, you not only can't dereference it, but you also shouldn't
> even compare pointer values - because the pointer arithmetic that was
> valid for loop entries is not valid for the HEAD entry that is
> embedded in another type. So the pointer arithmetic might have turned
> it into a pointer outside the real container of the HEAD, and might
> actually match something else.

Yes, this is what I had put together as well about this patch, and I
think it is OK as-is. In this case the list head is in the .bss of a
module so I don't think it is very likely that the type confused
container_of() will alias a kalloc result, but it is certainly
technically wrong as-is.

> So elsewhere I suggested that the fix to "you can't use the pointer
> outside the loop" be made to literally disallow it (using C99 for-loop
> variables seems the cleanest model), and have the compiler refuse to
> touch code that tries to use the loop iterator outside.

Oh yes, that would be really nice solution.

Jason