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Message-ID: <20190130182157.GP18558@fuggles.cambridge.arm.com>
Date: Wed, 30 Jan 2019 18:21:57 +0000
From: Will Deacon <will.deacon@....com>
To: Jann Horn <jannh@...gle.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
linux-arm-kernel@...ts.infradead.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
kernel list <linux-kernel@...r.kernel.org>
Subject: Re: ARM64 suggestion: reduce the compat address limit (currently
0x100000000)?
Hi Jann,
On Wed, Jan 30, 2019 at 05:32:00PM +0100, Jann Horn wrote:
> At the moment, compat tasks running on ARM64 can allocate memory up to
> 0x100000000 (TASK_SIZE_32). Testing on an Android device (with an
> admittedly somewhat old kernel):
[...]
> ffff1000-100000000 rw-p 00000000 00:00 0
>
> This means that mmap() allocations do not adhere to section 6.5.8 of
> C99 ("If the
> expression P points to an element of an array object and the
> expression Q points to the
> last element of the same array object, the pointer expression Q+1
> compares greater than
> P.") if you treat mmap() allocations as returning an array.
Oh, good point.
> In practice, I've also seen code that does things like computing a
> pointer that is out of bounds by a few bytes and then comparing it
> against the end of the array; while this is UB according to C99, it
> probably makes sense to try to avoid breaking such code.
Agreed, and since the current behaviour isn't something you can portably
rely on anyway, I think we're ok to change it.
> X86-64's compat code uses the limit 0xFFFFe000 (IA32_PAGE_OFFSET),
> which I think makes more sense. Would it make sense to do something
> like the following (untested)?
Can't we just go with 0x100000000 - PAGE_SIZE instead?
Will
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