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

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Date:   Sun, 9 Jul 2017 07:11:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, izumi.taku@...fujitsu.com,
        fanc.fnst@...fujitsu.com, Thomas Garnier <thgarnie@...gle.com>
Subject: Re: [PATCH v4 4/4] x86/boot/KASLR: Restrict kernel to be randomized
 in mirror regions

On Sun, Jul 9, 2017 at 5:37 AM, Baoquan He <bhe@...hat.com> 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@...hat.com>
> ---
>  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:


> +
> +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