kernel-hardening - Re: [PATCH v4 02/10] asm/nospec, array_ptr: sanitize speculative array de-references

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Message-ID: <CAPcyv4jUDTGpWm-ATMo=3P7TJxxtCK7BBo3KWsmUuuzxHMDmug@mail.gmail.com>
Date: Fri, 19 Jan 2018 12:55:08 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jann Horn <jannh@...gle.com>, kernel list <linux-kernel@...r.kernel.org>, 
	linux-arch <linux-arch@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Catalin Marinas <catalin.marinas@....com>, "the arch/x86 maintainers" <x86@...nel.org>, 
	Will Deacon <will.deacon@....com>, Russell King <linux@...linux.org.uk>, 
	Ingo Molnar <mingo@...hat.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Andrew Morton <akpm@...ux-foundation.org>, Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v4 02/10] asm/nospec, array_ptr:
 sanitize speculative array de-references

On Fri, Jan 19, 2018 at 10:18 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jan 19, 2018 at 2:20 AM, Jann Horn <jannh@...gle.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.

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