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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 15 Feb 2018 14:03:51 -0800
Message-ID: <CA+55aFz7g4wuwgc7hNFfYf+OqgPax3-F=yz8Xar7iemTin=FEg@mail.gmail.com>
Subject: Re: [PATCH] linux/nospec.h: allow index argument to have
 const-qualified type
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        stable <stable@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Feb 15, 2018 at 1:56 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>
> So I don't mind removing it, but I don't think it is garbage. It's
> there purely as a notification to the odd kernel developer that wants
> to pass "insane" index values,

But the thing is, the "index" value isn't even kernel-supplied.

Here's a test:  run a 32-bit kernel, and then do an ioctl() or
something with a negative fd.

What I think will happen is:

 - the negative fd will be seen as a big 'unsigned int' here:

        fcheck_files(struct files_struct *files, unsigned int fd)

which then does

                fd = array_index_nospec(fd, fdt->max_fds);

and that existing *STUPID* and *WRONG* WARN_ON() will trigger.

Sure, you can't trigger it on 64-bit kernels because there the
"unsigned int" will be small compared to LONG_MAX, but..

It is simply is *wrong* to check the "index".  It really fundamentally
is complete garbage.

Because the whole - and ONLY - *point* of this is that you have an
untrusted index. So checking it and giving a warning when it's out of
range is pure garbage.

Really. That warning must go away. Stop arguing for it, it's stupid and wrong.

Checking _size_ is one thing, but honestly, that's questionable too.

                  Linus