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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 19 Jan 2018 10:18:53 -0800
Message-ID: <CA+55aFw-RNg4diZ0RCVkEYAANDp22YDskNOR913vKQEBdarJSw@mail.gmail.com>
Subject: Re: [kernel-hardening] [PATCH v4 02/10] asm/nospec, array_ptr:
 sanitize speculative array de-references
To:     Jann Horn <jannh@google.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alan Cox <alan@linux.intel.com>
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On Fri, Jan 19, 2018 at 2:20 AM, Jann Horn <jannh@google.com> wrote:
>> +                                                                       \
>> +       __u._ptr = _arr + (_i & _mask);                                 \
>> +       __u._bit &= _mask;                                              \
>
> AFAICS, if `idx` is out of bounds, you first zero out the index
> (`_i & _mask`) and then immediately afterwards zero out
> the whole pointer (`_u._bit &= _mask`).
> Is there a reason for the `_i & _mask`, and if so, can you
> add a comment explaining that?

I think that's just leftovers from my original (untested) thing that
also did the access itself. So that __u._bit masking wasn't masking
the pointer, it was masking the value that was *read* from the
pointer, so that you could know that an invalid access returned
0/NULL, not just the first value in the array.

                  Linus