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References: <20210213012316.1525419-1-pasha.tatashin@soleen.com> <06b7bfd1-99cd-a9be-e3cc-9fe13f2cf2a6@arm.com>
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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Mon, 15 Feb 2021 08:42:17 -0500
Message-ID: <CA+CK2bBvaSzfD_mN3HdBdLrvQ3XDEPY0o2J8Ho8sViWX2apWyA@mail.gmail.com>
Subject: Re: [PATCH] arm64: mm: correct the start of physical address in
 linear map
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     Tyler Hicks <tyhicks@linux.microsoft.com>,
        James Morris <jmorris@namei.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>, ardb@kernel.org,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Feb 15, 2021 at 12:26 AM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> Hello Pavel,
>
> On 2/13/21 6:53 AM, Pavel Tatashin wrote:
> > Memory hotplug may fail on systems with CONFIG_RANDOMIZE_BASE because the
> > linear map range is not checked correctly.
> >
> > The start physical address that linear map covers can be actually at the
> > end of the range because of randmomization. Check that and if so reduce it
> > to 0.
>
> Looking at the code, this seems possible if memstart_addr which is a signed
> value becomes large (after falling below 0) during arm64_memblock_init().

Right.

>
> >
> > This can be verified on QEMU with setting kaslr-seed to ~0ul:
> >
> > memstart_offset_seed = 0xffff
> > START: __pa(_PAGE_OFFSET(vabits_actual)) = ffff9000c0000000
> > END:   __pa(PAGE_END - 1) =  1000bfffffff
> >
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> > Fixes: 58284a901b42 ("arm64/mm: Validate hotplug range before creating linear mapping")
> > ---
> >  arch/arm64/mm/mmu.c | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index ae0c3d023824..6057ecaea897 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -1444,14 +1444,25 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
> >
> >  static bool inside_linear_region(u64 start, u64 size)
> >  {
> > +     u64 start_linear_pa = __pa(_PAGE_OFFSET(vabits_actual));
> > +     u64 end_linear_pa = __pa(PAGE_END - 1);
> > +
> > +     /*
> > +      * Check for a wrap, it is possible because of randomized linear mapping
> > +      * the start physical address is actually bigger than the end physical
> > +      * address. In this case set start to zero because [0, end_linear_pa]
> > +      * range must still be able to cover all addressable physical addresses.
> > +      */
>
> If this is possible only with randomized linear mapping, could you please
> add IS_ENABLED(CONFIG_RANDOMIZED_BASE) during the switch over. Wondering
> if WARN_ON(start_linear_pa > end_linear_pa) should be added otherwise i.e
> when linear mapping randomization is not enabled.

Yeah, good idea, I will add ifdef for CONFIG_RANDOMIZED_BASE.

>
> > +     if (start_linear_pa > end_linear_pa)
> > +             start_linear_pa = 0;
>
> This looks okay but will double check and give it some more testing.

Thank you,
Pasha