Re: [PATCH] x86: fix kaslr and memmap collision

[Kees Cook]

On Tue, Nov 22, 2016 at 11:01 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Tue, Nov 22, 2016 at 10:54 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Nov 22, 2016 at 9:26 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>> [ replying for Dave since he's offline today and tomorrow ]
>>>
>>> On Tue, Nov 22, 2016 at 12:47 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>>>
>>>> * Dave Jiang <dave.jiang@xxxxxxxxx> wrote:
>>>>
>>>>> CONFIG_RANDOMIZE_BASE relocates the kernel to a random base address.
>>>>> However it does not take into account the memmap= parameter passed in from
>>>>> the kernel commandline.
>>>>
>>>> memmap= parameters are often used as a list.
>>>>
>>>>> [...] This results in the kernel sometimes being put in the middle of the user
>>>>> memmap. [...]
>>>>
>>>> What does this mean? If memmap= is used to re-define the memory map then the
>>>> kernel getting in the middle of a RAM area is what we want, isn't it? What we
>>>> don't want is for the kernel to get into reserved areas, right?
>>>
>>> Right, this is about teaching kaslr to not land the kernel in newly
>>> defined reserved regions that were not marked reserved in the initial
>>> e820 map from platform firmware.
>>>
>>>>> [...] Check has been added in the kaslr in order to avoid the region marked by
>>>>> memmap.
>>>>
>>>> What does this mean?
>>>
>>> Is this clearer? "Update the set of 'mem_avoid' entries to exclude
>>> 'memmap=' defined reserved regions from the set of valid address range
>>> to land the kernel image."
>>>
>>>>
>>>>> Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
>>>>> ---
>>>>>  arch/x86/boot/boot.h             |    2 ++
>>>>>  arch/x86/boot/compressed/kaslr.c |   45 ++++++++++++++++++++++++++++++++++++++
>>>>>  arch/x86/boot/string.c           |   25 +++++++++++++++++++++
>>>>>  3 files changed, 72 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
>>>>> index e5612f3..0d5fe5b 100644
>>>>> --- a/arch/x86/boot/boot.h
>>>>> +++ b/arch/x86/boot/boot.h
>>>>> @@ -332,6 +332,8 @@ int strncmp(const char *cs, const char *ct, size_t count);
>>>>>  size_t strnlen(const char *s, size_t maxlen);
>>>>>  unsigned int atou(const char *s);
>>>>>  unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base);
>>>>> +unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base);
>>>>> +long simple_strtol(const char *cp, char **endp, unsigned int base);
>>>>>  size_t strlen(const char *s);
>>>>>
>>>>>  /* tty.c */
>>>>> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
>>>>> index a66854d..6fb8f1ec 100644
>>>>> --- a/arch/x86/boot/compressed/kaslr.c
>>>>> +++ b/arch/x86/boot/compressed/kaslr.c
>>>>> @@ -11,6 +11,7 @@
>>>>>   */
>>>>>  #include "misc.h"
>>>>>  #include "error.h"
>>>>> +#include "../boot.h"
>>>>>
>>>>>  #include <generated/compile.h>
>>>>>  #include <linux/module.h>
>>>>> @@ -61,6 +62,7 @@ enum mem_avoid_index {
>>>>>       MEM_AVOID_INITRD,
>>>>>       MEM_AVOID_CMDLINE,
>>>>>       MEM_AVOID_BOOTPARAMS,
>>>>> +     MEM_AVOID_MEMMAP,
>>>>>       MEM_AVOID_MAX,
>>>>>  };
>>>>>
>>>>> @@ -77,6 +79,37 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
>>>>>       return true;
>>>>>  }
>>>>>
>>>>> +#include "../../../../lib/cmdline.c"
>>>>> +
>>>>> +static int
>>>>> +parse_memmap(char *p, unsigned long long *start, unsigned long long *size)
>>>>> +{
>>>>> +     char *oldp;
>>>>> +
>>>>> +     if (!p)
>>>>> +             return -EINVAL;
>>>>> +
>>>>> +     /* we don't care about this option here */
>>>>> +     if (!strncmp(p, "exactmap", 8))
>>>>> +             return -EINVAL;
>>>>> +
>>>>> +     oldp = p;
>>>>> +     *size = memparse(p, &p);
>>>>> +     if (p == oldp)
>>>>> +             return -EINVAL;
>>>>> +
>>>>> +     switch (*p) {
>>>>> +     case '@':
>>>>> +     case '#':
>>>>> +     case '$':
>>>>> +     case '!':
>>>>> +             *start = memparse(p+1, &p);
>>>>> +             return 0;
>>>>> +     }
>>>>> +
>>>>> +     return -EINVAL;
>>>>> +}
>>>>> +
>>>>>  /*
>>>>>   * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
>>>>>   * The mem_avoid array is used to store the ranges that need to be avoided
>>>>> @@ -158,6 +191,8 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
>>>>>       u64 initrd_start, initrd_size;
>>>>>       u64 cmd_line, cmd_line_size;
>>>>>       char *ptr;
>>>>> +     char arg[38];
>>>>
>>>> Where does the magic '38' come from?
>>>>
>>>>> +     unsigned long long memmap_start, memmap_size;
>>>>>
>>>>>       /*
>>>>>        * Avoid the region that is unsafe to overlap during
>>>>> @@ -195,6 +230,16 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
>>>>>       add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start,
>>>>>                        mem_avoid[MEM_AVOID_BOOTPARAMS].size);
>>>>>
>>>>> +     /* see if we have any memmap areas */
>>>>> +     if (cmdline_find_option("memmap", arg, sizeof(arg)) > 0) {
>>>>> +             int rc = parse_memmap(arg, &memmap_start, &memmap_size);
>>>>> +
>>>>> +             if (!rc) {
>>>>> +                     mem_avoid[MEM_AVOID_MEMMAP].start = memmap_start;
>>>>> +                     mem_avoid[MEM_AVOID_MEMMAP].size = memmap_size;
>>>>> +             }
>>>>> +     }
>>>>> +
>>>>
>>>> This only handles a single (first) memmap argument, is that sufficient?
>>>
>>> No, you're right, we need to handle multiple ranges.  Since the
>>> mem_avoid array is statically allocated perhaps we can handle up to 4
>>> memmap= entries, but past that point disable kaslr for that boot?
>>
>> Yeah, that seems fine to me. I assume it's rare to have 4?
>>
>
> It should be rare to have *one* since ACPI 6.0 added support for
> communicating persistent memory ranges.  However there are legacy
> nvdimm users that I know are doing at least 2, but I have hard time
> imagining they would ever do more than 4.

Cool. As long as it announces KASLR being disabled (as in some of the
other conditions) that should be fine.

-Kees

-- 
Kees Cook
Nexus Security