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Date: Tue, 22 Nov 2016 11:01:32 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Kees Cook <keescook@...omium.org>
Cc: Ingo Molnar <mingo@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
david <david@...morbit.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] x86: fix kaslr and memmap collision
On Tue, Nov 22, 2016 at 10:54 AM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Nov 22, 2016 at 9:26 AM, Dan Williams <dan.j.williams@...el.com> wrote:
>> [ replying for Dave since he's offline today and tomorrow ]
>>
>> On Tue, Nov 22, 2016 at 12:47 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>>
>>> * Dave Jiang <dave.jiang@...el.com> 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@...el.com>
>>>> ---
>>>> 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.
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