Re: [PATCH resend] x86, PAT: honor CONFIG_STRICT_DEVMEM if pat is disabled

[Venki Pallipadi]

On Fri, Jul 22, 2011 at 2:01 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
>
> since 0124cecf (x86, PAT: disable /dev/mem mmap RAM with PAT) we are
> disabling access to the system RAM as if STRICT_DEVMEM was enabled by
> default. If we, however, disable pat by nopat kernel parameter we can
> access that memory without considering CONFIG_STRICT_DEVMEM.

Looking at the code, I don't think this problem exists. Code in pat.c
disables /dev/mem mmap only when !CONFIG_STRICT_DEVMEM.

When CONFIG_STRICT_DEVMEM, irrespective of pat or nopat, code in mem.c
disables mmap access. The code in pat.c has no effect on whether mmap
is allowed or not.

When !CONFIG_STRICT_DEVMEM, mmap is disabled with pat. And on nopat
mmap is allowed with the check in pat.c:range_is_allowed() with the
change from commit 9e41bff2.

Thanks,
Venki

>
> Let's make the code to honor STRICT_DEVMEM while we still preserve
> disabled by default behavior. This means that the access to the system
> memory is granted only if nopat is provided and STRICT_DEVMEM is
> disabled.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> ---
>  arch/x86/mm/pat.c |   10 ++--------
>  1 files changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index f6ff57b..fbaf3fc 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -472,13 +472,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>        return vma_prot;
>  }
>
> -#ifdef CONFIG_STRICT_DEVMEM
> -/* This check is done in drivers/char/mem.c in case of STRICT_DEVMEM*/
> -static inline int range_is_allowed(unsigned long pfn, unsigned long size)
> -{
> -       return 1;
> -}
> -#else
>  /* This check is needed to avoid cache aliasing when PAT is enabled */
>  static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>  {
> @@ -486,8 +479,10 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>        u64 to = from + size;
>        u64 cursor = from;
>
> +#ifndef CONFIG_STRICT_DEVMEM
>        if (!pat_enabled)
>                return 1;
> +#endif
>
>        while (cursor < to) {
>                if (!devmem_is_allowed(pfn)) {
> @@ -501,7 +496,6 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>        }
>        return 1;
>  }
> -#endif /* CONFIG_STRICT_DEVMEM */
>
>  int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
>                                unsigned long size, pgprot_t *vma_prot)
> --
> 1.7.5.4
>
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