Messages in this thread | | | Date | Tue, 26 Oct 2021 12:15:58 +0100 | From | "Russell King (Oracle)" <> | Subject | Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans multiple pmds |
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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@armlinux.org.uk> 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@windriver.com> 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@windriver.com> wrote: > > > > > > > > > > > > > > > From: Quanyang Wang <quanyang.wang@windriver.com> > > > > > > > > > > > > > > > > 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@windriver.com> > > > > > > > > > > > > > > Looks reasonable to me. > > > > > > > Reviewed-by: Linus Walleij <linus.walleij@linaro.org> > > > > > > > > > > > > > > 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.
-- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
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