Re: [PATCH 3/4] KASLR: Handle memory limit specified by memmap and mem option

[Kees Cook]

On Mon, Apr 17, 2017 at 6:34 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Option mem= will limit the max address system can use. Any memory
> region above the limit will be removed. And memmap=nn[KMG] which
> has no offset specified has the same behaviour as mem=. KASLR need
> consider this when choose the random position for decompressing
> kernel. Do it in this patch.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> ---
>  arch/x86/boot/compressed/kaslr.c | 53 +++++++++++++++++++++++++++++++---------
>  1 file changed, 42 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 36ab429..5361abd 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -67,6 +67,10 @@ int mem_avoid_memmap_index;
>  extern unsigned long get_cmd_line_ptr(void);
>
>
> +/* Store memory limit specified by "mem=nn[KMG]" or "memmap=nn[KMG]" */
> +unsigned long long mem_limit = ULLONG_MAX;

I would either make this 0 or ULLONG_MAX - 1 (see below).

> +
> +
>  enum mem_avoid_index {
>         MEM_AVOID_ZO_RANGE = 0,
>         MEM_AVOID_INITRD,
> @@ -117,15 +121,18 @@ parse_memmap(char *p, unsigned long long *start, unsigned long long *size)
>                 return -EINVAL;
>
>         switch (*p) {
> -       case '@':
> -               /* Skip this region, usable */
> -               *size = 0;
> -               *start = 0;
>         case '#':
>         case '$':
>         case '!':
>                 *start = memparse(p + 1, &p);
>                 return 0;
> +       case '@':
> +               /* Skip this region, usable */
> +               *size = 0;

Now it looks like we're intentionally falling through. A comment
should be included to indicate it.

> +       default:
> +               /* Avoid the region which is above the amount limit */
> +               *start = 0;
> +               return 0;
>         }
>
>         return -EINVAL;
> @@ -151,9 +158,14 @@ static void mem_avoid_memmap(char *str)
>                 if (rc < 0)
>                         break;
>                 str = k;
> -               /* A usable region that should not be skipped */
> -               if (size == 0)
> +
> +               if (start == 0) {
> +                       /* Store the specified memory limit if size > 0 */
> +                       if (size > 0)
> +                               mem_limit = size;
> +
>                         continue;
> +               }
>
>                 mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].start = start;
>                 mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
> @@ -173,6 +185,7 @@ static int handle_mem_memmap(void)
>         char tmp_cmdline[COMMAND_LINE_SIZE];
>         size_t len = strlen((char *)args);
>         char *param, *val;
> +       u64 mem_size;
>
>         len = (len >= COMMAND_LINE_SIZE) ? COMMAND_LINE_SIZE - 1 : len;
>         memcpy(tmp_cmdline, args, len);
> @@ -195,8 +208,18 @@ static int handle_mem_memmap(void)
>                         return -1;
>                 }
>
> -               if (!strcmp(param, "memmap"))
> +               if (!strcmp(param, "memmap")) {
>                         mem_avoid_memmap(val);
> +               } else if (!strcmp(param, "mem")) {
> +                       char *p = val;
> +
> +                       if (!strcmp(p, "nopentium"))
> +                               continue;
> +                       mem_size = memparse(p, &p);
> +                       if (mem_size == 0)
> +                               return -EINVAL;
> +                       mem_limit = mem_size;
> +               }
>         }
>
>         return 0;
> @@ -432,7 +455,8 @@ static void process_e820_entry(struct e820entry *entry,
>  {
>         struct mem_vector region, overlap;
>         struct slot_area slot_area;
> -       unsigned long start_orig;
> +       unsigned long start_orig, end;
> +       struct e820entry cur_entry;
>
>         /* Skip non-RAM entries. */
>         if (entry->type != E820_RAM)
> @@ -446,8 +470,15 @@ static void process_e820_entry(struct e820entry *entry,
>         if (entry->addr + entry->size < minimum)
>                 return;
>
> -       region.start = entry->addr;
> -       region.size = entry->size;
> +       /* Ignore entries above memory limit */
> +       end = min(entry->size + entry->addr - 1, mem_limit);
> +       if (entry->addr >= end)
> +               return;
> +       cur_entry.addr = entry->addr;
> +       cur_entry.size = end - entry->addr + 1;
> +
> +       region.start = cur_entry.addr;
> +       region.size = cur_entry.size;

I find the manipulation of entry->addr +/- 1 confusing; it should just
be mem_limit that is adjusted:

    end = min(entry->size + entry->addr, mem_limit + 1);

And maybe to avoid mem_limit being giant by default, maybe have "0" be special?

    cur_entry.addr = entry->addr;
    if (mem_limit) {
         unsigned long end = min(entry->size + entry->addr, mem_limit + 1);
         if (entry->addr > end)
             return;
         cur_entry.size = end - entry->addr;
    } else {
          cur_entry.size = entry->size;
    }

or something... and maybe move the whole thing earlier so other tests
that examine entry->size are checked with the new adjusted value.

-Kees

>
>         /* Give up if slot area array is full. */
>         while (slot_area_index < MAX_SLOT_AREA) {
> @@ -461,7 +492,7 @@ static void process_e820_entry(struct e820entry *entry,
>                 region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
>
>                 /* Did we raise the address above this e820 region? */
> -               if (region.start > entry->addr + entry->size)
> +               if (region.start > cur_entry.addr + cur_entry.size)
>                         return;
>
>                 /* Reduce size by any delta from the original address. */
> --
> 2.5.5
>



-- 
Kees Cook
Pixel Security