linux-kernel - Re: [PATCH] linux/nospec.h: allow index argument to have const-qualified type

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Message-ID: <CAPcyv4iDDBWD09b5Pm5=D8+EKZQpMfK4zs1dGfptf5Sf8sSdJA@mail.gmail.com>
Date:   Thu, 15 Feb 2018 14:08:41 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        stable <stable@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/nospec.h: allow index argument to have
 const-qualified type

On Thu, Feb 15, 2018 at 2:03 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Feb 15, 2018 at 1:56 PM, Dan Williams <dan.j.williams@...el.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.

True, I had been myopically focused on the 64-bit case.

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

Nah, I'm not going to argue for that.