linux-kernel - Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region

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Date:   Fri, 13 Nov 2020 08:06:13 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Steve Capper <steve.capper@....com>,
        Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...nel.org>,
        gshan@...hat.com, Robin Murphy <robin.murphy@....com>,
        Steven Price <steven.price@....com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] arm64: mm: account for hotplug memory when randomizing
 the linear region

On Fri, 13 Nov 2020 at 08:03, Anshuman Khandual
<anshuman.khandual@....com> wrote:
>
>
>
> On 11/13/20 11:44 AM, Ard Biesheuvel wrote:
> > On Fri, 13 Nov 2020 at 04:16, Anshuman Khandual
> > <anshuman.khandual@....com> wrote:
> >>
> >>
> >>
> >> On 11/12/20 2:55 PM, Catalin Marinas wrote:
> >>> Hi Anshuman,
> >>>
> >>> On Wed, Nov 11, 2020 at 09:18:56AM +0530, Anshuman Khandual wrote:
> >>>> On 11/11/20 12:44 AM, Catalin Marinas wrote:
> >>>>> On Wed, 14 Oct 2020 10:18:57 +0200, Ard Biesheuvel wrote:
> >>>>>> As a hardening measure, we currently randomize the placement of
> >>>>>> physical memory inside the linear region when KASLR is in effect.
> >>>>>> Since the random offset at which to place the available physical
> >>>>>> memory inside the linear region is chosen early at boot, it is
> >>>>>> based on the memblock description of memory, which does not cover
> >>>>>> hotplug memory. The consequence of this is that the randomization
> >>>>>> offset may be chosen such that any hotplugged memory located above
> >>>>>> memblock_end_of_DRAM() that appears later is pushed off the end of
> >>>>>> the linear region, where it cannot be accessed.
> >>>>>>
> >>>>>> [...]
> >>>>>
> >>>>> Applied to arm64 (for-next/mem-hotplug), thanks!
> >>>>>
> >>>>> [1/1] arm64: mm: account for hotplug memory when randomizing the linear region
> >>>>>       https://git.kernel.org/arm64/c/97d6786e0669
> >>>>
> >>>> Got delayed and never made here in time, sorry about that. Nonetheless,
> >>>> I have got something working with respect to the generic mechanism that
> >>>> David Hildenbrand had asked for earlier.
> >>>>
> >>>> https://patchwork.kernel.org/project/linux-arm-kernel/patch/1600332402-30123-1-git-send-email-anshuman.khandual@arm.com/
> >>>
> >>> There was a lot of discussion around this patch but I haven't seen any
> >>> new version posted.
> >>
> >> Just posted before some time.
> >>
> >> https://lore.kernel.org/linux-arm-kernel/1605236574-14636-1-git-send-email-anshuman.khandual@arm.com/
> >>
> >
> > You failed to cc me on that patch.
>
> I could see 'ardb@...nel.org' marked as a copy on the patch. You
> did not receive the email ? The CC list is in the commit message
> itself. Even the lore.kernel.org based URL does list you email
> as well. Not sure what might have happened.
>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Steven Price <steven.price@....com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
>

Right. Not sure what happened there, I may have deleted it by
accident. Apologies.

> >
> > The logic looks correct but please fix up the comment block:
> > - PAGE_END is no longer defined in terms of vabits_actual
> > - bits [51..48] are not ignored by the MMU
> >
> > Actually, I think the entire second paragraph of that comment block
> > can be dropped.
>
> And from the commit message as well, had reused it in both places.
>
> >
> > Please also fix up the coding style:
> > - put && at the end of the first line
> > - drop the redundant parens
> > - fix the indentation
>
> Does this look okay ?
>
> static bool inside_linear_region(u64 start, u64 size)
> {
>         /*
>          * Linear mapping region is the range [PAGE_OFFSET..(PAGE_END - 1)]
>          * accommodating both its ends but excluding PAGE_END. Max physical
>          * range which can be mapped inside this linear mapping range, must
>          * also be derived from its end points.
>          */
>         return start >= __pa(_PAGE_OFFSET(vabits_actual)) &&
>                         (start + size - 1) <= __pa(PAGE_END - 1);
> }

Not sure whether the whitespace has been mangled by the email client,
but the first ( on the second line should align vertically with the
's' of 'start' on the first line