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

Kees Cook
2017-04-18T20:36:43Z

On Mon, Apr 17, 2017 at 6:34 AM, Baoquan He <bhe@redhat.com> 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@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: x86@kernel.org
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Yinghai Lu <yinghai@kernel.org>
> Cc: Borislav Petkov <bp@suse.de>
> ---
> 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