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From: Dan Williams <dan.j.williams@intel.com>
Date: Fri, 19 Jan 2018 12:55:08 -0800
Message-ID: <CAPcyv4jUDTGpWm-ATMo=3P7TJxxtCK7BBo3KWsmUuuzxHMDmug@mail.gmail.com>
Subject: Re: [kernel-hardening] [PATCH v4 02/10] asm/nospec, array_ptr:
 sanitize speculative array de-references
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jann Horn <jannh@google.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 10:18 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> 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.

Yes, the index masking can be dropped since we're returning a
sanitized array element pointer now.