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Date: Sun, 21 Jan 2018 21:15:38 +0100
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 <linux-arch@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>, "the arch/x86 maintainers" <x86@...nel.org>,
Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "H. Peter Anvin" <hpa@...or.com>,
Linus Torvalds <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
[whoops, resending as non-HTML mail]
On Sun, Jan 21, 2018 at 11:40 AM, 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?
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