Re: [PATCH v4 4/4] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

[Baoquan He]

On 07/09/17 at 07:11am, Kees Cook wrote:
> On Sun, Jul 9, 2017 at 5:37 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> > Kernel text may be located in non-mirror regions (movable zone) when both
> > address range mirroring feature and KASLR are enabled.
> >
> > The address range mirroring feature arranges such mirror region into
> > normal zone and other region into movable zone in order to locate
> > kernel code and data in mirror region. The physical memory region
> > whose descriptors in EFI memory map has EFI_MEMORY_MORE_RELIABLE
> > attribute (bit: 16) are mirrored.
> >
> > If efi is detected, iterate efi memory map and pick the mirror region to
> > process for adding candidate of randomization slot. If efi is disabled
> > or no mirror region found, still process e820 memory map.
> >
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >  arch/x86/boot/compressed/kaslr.c | 60 ++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 58 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 99c7194f7ea6..7376b3473758 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -37,7 +37,9 @@
> >  #include <linux/uts.h>
> >  #include <linux/utsname.h>
> >  #include <linux/ctype.h>
> > +#include <linux/efi.h>
> >  #include <generated/utsrelease.h>
> > +#include <asm/efi.h>
> >
> >  /* Macros used by the included decompressor code below. */
> >  #define STATIC
> > @@ -558,6 +560,54 @@ static void process_mem_region(struct mem_vector *entry,
> >         }
> >  }
> >
> > +/* Marks if efi mirror regions have been found and handled. */
> > +static bool efi_mirror_found;
> 
> I think this is only ever checked once? How about having
> process_efi_entries return bool to indicate if mirror was found? Also,
> that function should be behind #ifdef. Let's do something like this:

Yes. Yours looks much better, let me change as you suggested and repost.
Thanks a lot for reviewing and great suggestions!

> 
> 
> > +
> > +static void process_efi_entries(unsigned long minimum,
> > +                               unsigned long image_size)
> 
> #ifdef CONFIG_EFI
> /* Returns true if mirror region found (and further scanning should stop) */
> static bool process_efi_entries(...)
> {
> ...
> }
> #else
> static inline bool process_efi_entries(...)
> {
>     return false;
> }
> #endif
> 
> Then:
> 
> > +#ifdef CONFIG_EFI
> > +       process_efi_entries(minimum, image_size);
> > +       if (efi_mirror_found)
> > +               return slots_fetch_random();
> > +#endif
> 
> Can become:
> 
> if (process_efi_entries(minimum, image_size))
>     return slots_fetch_random()
> 
> and no #ifndef needed here.
> 
> > +
> >         process_e820_entries(minimum, image_size);
> >         return slots_fetch_random();
> >  }
> > @@ -652,7 +708,7 @@ void choose_random_location(unsigned long input,
> >          */
> >         min_addr = min(*output, 512UL << 20);
> >
> > -       /* Walk e820 and find a random address. */
> > +       /* Walk available memory entries to find a random address. */
> >         random_addr = find_random_phys_addr(min_addr, output_size);
> >         if (!random_addr) {
> >                 warn("Physical KASLR disabled: no suitable memory region!");
> > --
> > 2.5.5
> >
> 
> Otherwise, if the EFI logic is good, this looks sensible.
> 
> Thanks for splitting up the patches!
> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security