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

[Russell King (Oracle)]

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:
> > > > >
> > > > > Hi,
> > > > > Sorry for the inconvenience.
> > > > >
> > > > > On 10/26/21 4:59 PM, Russell King (Oracle) wrote:
> > > > > > On Sun, Oct 24, 2021 at 11:44:31PM +0200, Linus Walleij wrote:
> > > > > > > On Wed, Oct 20, 2021 at 7:50 AM <quanyang.wang@xxxxxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > > From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> > > > > > > >
> > > > > > > > Not only the early fixmap range, but also the fixmap range should be
> > > > > > > > checked if it spans multiple pmds. When enabling CONFIG_DEBUG_HIGHMEM,
> > > > > > > > some systems which contain up to 16 CPUs will crash.
> > > > > > > >
> > > > > > > > Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> > > > > > >
> > > > > > > Looks reasonable to me.
> > > > > > > Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> > > > > > >
> > > > > > > Please submit this patch into Russell's patch tracker.
> > > > > >
> > > > > > ... and has totally broken what looks like _all_ ARM kernel builds.
> > > > > This patch is intended to trigger build error when it check the value of
> > > > > __end_of_fixmap_region is equal or larger than 256.
> > > >
> > > > Why? The fixmap region is larger than one PMD, so why do we need to cap it?
> > > In __kmap_local_pfn_prot, arch_kmap_local_set_pte(&init_mm, vaddr, kmap_pte
> > > - idx, pteval) is used to set pteval.
> > > 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.

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