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Date: Sun, 21 Jan 2018 15:07:57 +0000
From: Jann Horn <jannh@...gle.com>
To: Russell King - ARM Linux <linux@...linux.org.uk>
Cc: Dan Williams <dan.j.williams@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com,
Peter Zijlstra <peterz@...radead.org>, Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...hat.com>, gregkh@...uxfoundation.org,
"H. Peter Anvin" <hpa@...or.com>, torvalds@...ux-foundation.org, alan@...ux.intel.com
Subject: Re: Re: [PATCH v4.1 02/10] asm/nospec, array_ptr:
sanitize speculative array de-references
On Sun, Jan 21, 2018, 11:40 Russell King - ARM Linux <linux@...linux.org.uk>
wrote:
> On Sat, Jan 20, 2018 at 01:06:09PM -0800, Dan Williams wrote:
> > +/*
> > + * If idx is negative or if idx > size then bit 63 is set in the mask,
> > + * and the value of ~(-1L) is zero. When the mask is zero, bounds check
> > + * failed, array_ptr will return NULL.
>
> The more times I see this the more times I'm unhappy with this comment:
>
> 1. does this really mean "idx > size" or "idx >= size" ? The code
> implements the latter not the former.
>
Copying the code here for context:
return ~(long)(idx | (sz - 1 - idx)) >> (BITS_PER_LONG - 1);
That part of the condition (ignoring the overflow edgecases) is equivalent
to "!(idx > sz - 1)", which is equivalent to "idx <= sz - 1", which is
(ignoring overflow edgecases) equivalent to "idx < sz".
Handling of edgecases:
idx>=2^(BITS_PER_LONG-1) will cause a NULL return through the first part of
the condition.
Hmm... a problematic case might be "sz==0 && idx==2^(BITS_PER_LONG-1)-1".
The first part of the expression wouldn't trigger, the second part would be
"2^(BITS_PER_LONG)-1-(2^(BITS_PER_LONG-1)-1) ==
2^(BITS_PER_LONG)-2^(BITS_PER_LONG-1)
== 2^(BITS_PER_LONG-1)", which also wouldn't trigger, I think?
2. is "bit 63" relevant here - what if longs are 32-bit? "the top bit"
> or "the sign bit" would be better.
>
> 3. "and the value of ~(-1L) is zero." So does this mean that when
> 0 <= idx < size, somehow the rules of logic change and ~(-1L)
> magically becomes no longer zero!
>
> I'd suggest changing the description to something like:
>
> * If 0 <= idx < size, return a mask of ~0UL, otherwise return zero.
>
> or:
>
> * When idx is out of bounds (iow, is negative or idx >= sz), the sign
> * bit will be set. Extend the sign bit to all bits and invert, giving
> * a result of zero for an out of bounds idx, or ~0UL if within bounds.
>
> depending on how deeply you want to describe what's going on here.
>
> --
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps
> up
> According to speedtest.net: 8.21Mbps down 510kbps up
>
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