Re: [PATCH] x86/mm: Do not warn about PCI BIOS W+X mappings

[Kees Cook]

On Mon, Oct 8, 2018 at 12:53 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> PCI BIOS requires the BIOS area 0x0A0000-0x0FFFFFF to be mapped W+X for
> various legacy reasons. When CONFIG_DEBUG_WX is enabled, this triggers the
> WX warning, but this is misleading because the mapping is required and is
> not a result of an accidental oversight.
>
> Prevent the full warning when PCI BIOS is enabled and the detected WX
> mapping is in the BIOS area. Just emit a pr_warn() which denotes the
> fact. This is partially duplicating the info which the PCI BIOS code emits
> when it maps the area as executable, but that info is not in the context of
> the WX checking output.
>
> Reported-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> ---
>  arch/x86/mm/dump_pagetables.c |   35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
>
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -19,7 +19,9 @@
>  #include <linux/sched.h>
>  #include <linux/seq_file.h>
>  #include <linux/highmem.h>
> +#include <linux/pci.h>
>
> +#include <asm/e820/types.h>
>  #include <asm/pgtable.h>
>
>  /*
> @@ -241,6 +243,29 @@ static unsigned long normalize_addr(unsi
>         return (signed long)(u << shift) >> shift;
>  }
>
> +static void note_wx(struct pg_state *st)
> +{
> +       unsigned long npages;
> +
> +       npages = (st->current_address - st->start_address) / PAGE_SIZE;
> +
> +#ifdef CONFIG_PCI_BIOS
> +       /*
> +        * If PCI BIOS is enabled, the PCI BIOS area is forced to WX.
> +        * Inform about it, but avoid the warning.
> +        */
> +       if (pcibios_enabled && st->start_address >= PAGE_OFFSET + BIOS_BEGIN &&
> +           st->current_address <= PAGE_OFFSET + BIOS_END) {
> +               pr_warn_once("x86/mm: PCI BIOS W+X mapping %lu pages\n", npages);
> +               return;
> +       }
> +#endif
> +       /* Account the WX pages */
> +       st->wx_pages += npages;
> +       WARN_ONCE(1, "x86/mm: Found insecure W+X mapping at address %p/%pS\n",
> +                 (void *)st->start_address, (void *)st->start_address);

Probably we should just drop the "%p/" part. %pS should give us
everything we need and %p will be meaningless normally.

Otherwise, yes, this look good.

-Kees

-- 
Kees Cook
Pixel Security