Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans multiple pmds

[Russell King (Oracle)]

On Tue, Oct 26, 2021 at 01:26:05PM +0200, Ard Biesheuvel wrote:
> On Tue, 26 Oct 2021 at 13:16, Russell King (Oracle)
> <linux@xxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Oct 26, 2021 at 12:56:08PM +0200, Ard Biesheuvel wrote:
> > > On Tue, 26 Oct 2021 at 12:55, Russell King (Oracle)
> > > <linux@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 26, 2021 at 06:38:16PM +0800, Quanyang Wang wrote:
> > > > > Hi Ard,
> > > > >
> > > > > On 10/26/21 6:12 PM, Ard Biesheuvel wrote:
> > > > > > On Tue, 26 Oct 2021 at 11:53, Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx> wrote:
> ...
> > > > > But the ptep is calculated by "kmap_pte - idx", which means all ptes must be
> > > > > placed next to each other and no gaps. But for ARM, the ptes for the range
> > > > > "0xffe00000~0xfff00000" is not next to the ptes for the range
> > > > > "0xffc80000~0xffdfffff".
> > > > >
> > > > > When the idx is larger than 256, virtual address is in 0xffdxxxxx, access
> > > > > this address will crash since its pteval isn't set correctly.
> > > >
> > > > Thanks for the explanation.
> > > >
> > > > Sadly, this does seem to be correct. Even if the PTE tables are
> > > > located next to each other in memory, they _still_ won't be a
> > > > contiguous array of entries due to being interleaved with the Linux
> > > > PTE table and the hardware PTE table.
> > > >
> > > > Since the address range 0xffe00000-0xfff00000 is already half of one
> > > > PTE table containing 512 contiguous entries, we are limited to 256
> > > > fixmap PTEs maximum. If we have more than that we will start trampling
> > > > over memory below the PTE table _and_ we will start corrupting Linux
> > > > PTE entries in the 0xfff00000-0xffffffff range.
> > > >
> > > > I suspect this hasn't been seen because of a general lack of ARM
> > > > systems with more than 4 CPUs.
> > > >
> > >
> > > But doesn't that make it a kmap_local regression? Or do you think this
> > > issue existed before that as well?
> >
> > It definitely is a bug in tglx's kmap_local code, which assumes all
> > PTEs in the fixmap region are contiguously arranged.
> >
> > Looking back further, when local kmaps were handled in arch code, this
> > bug did /not/ exist. We used to get the PTE entry to update via:
> >
> >         unsigned long vaddr = __fix_to_virt(idx);
> >         pte_t *ptep = pte_offset_kernel(pmd_off_k(vaddr), vaddr);
> >
> > which later became:
> >
> >         pte_t *ptep = virt_to_kpte(vaddr);
> >
> > Both of which walk the page tables.
> >
> > So in summary a regression caused by converting ARM to kmap_local.
> >
> > I think we could fix it by providing our own arch_kmap_local_set_pte()
> > which ignores the ptep argument, and instead walks the page tables
> > using the vaddr argument.
> >
> 
> Removing all occurrences of 'kmap_pte - idx' and replacing them with
> virt_to_kpte() seems to do the trick. Unfortunately, these occur in
> other places as well, not only on the map path, so I doubt that
> overriding arch_kmap_local_set_pte will be sufficient.

Right. That's probably going to upset some folk if we make everyone
walk page tables, so we probably need to add overrideable macros
just like arch_kmap_local_set_pte() is... which feels rather yucky.

tglx - any opinions on how you'd like this regression to be fixed?

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