Re: [PATCH] arm64: mm: correct the start of physical address in linear map

[Pavel Tatashin]

On Mon, Feb 15, 2021 at 12:26 AM Anshuman Khandual
<anshuman.khandual@xxxxxxx> 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@xxxxxxxxxx>
> > 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