[PATCH] arm64: mm: fix thinko in non-global page table attribute check

[Ard Biesheuvel]

(corrected Nicolas's email address - beware when replying)

On 23 February 2018 at 18:04, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> The routine pgattr_change_is_safe() was extended in commit 4e6020565596
> ("arm64: mm: Permit transitioning from Global to Non-Global without BBM")
> to permit changing the nG attribute from not set to set, but did so in a
> way that inadvertently disallows such changes if other permitted attribute
> changes take place at the same time. So update the code to take this into
> account.
>
> Fixes: 4e6020565596 ("arm64: mm: Permit transitioning from Global to ...")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/mm/mmu.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 451f96f3377c..5bdc2c4db9ad 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -107,7 +107,7 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
>          * The following mapping attributes may be updated in live
>          * kernel mappings without the need for break-before-make.
>          */
> -       static const pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE;
> +       static const pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE | PTE_NG;
>
>         /* creating or taking down mappings is always safe */
>         if (old == 0 || new == 0)
> @@ -117,9 +117,9 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
>         if ((old | new) & PTE_CONT)
>                 return false;
>
> -       /* Transitioning from Global to Non-Global is safe */
> -       if (((old ^ new) == PTE_NG) && (new & PTE_NG))
> -               return true;
> +       /* Transitioning from Non-Global to Global is unsafe */
> +       if (old & ~new & PTE_NG)
> +               return false;
>
>         return ((old ^ new) & ~mask) == 0;
>  }
> --
> 2.11.0
>