Re: [PATCH 5/6] x86: kaslr: select memory region from e820 maps

From: Kees Cook
Date: Fri Apr 26 2013 - 18:01:45 EST

On Fri, Apr 26, 2013 at 2:51 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Fri, Apr 26, 2013 at 12:03 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> This chooses the largest contiguous RAM region for the KASLR offset
>> to live in.
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>> v2:
>> - make sure to exclude e820 regions outside the 32-bit memory range.
> Do you need to execlude range that is used for initrd and possible
> command_line and boot_param ?

Yeah, and while doing a stress test here, I realized there's another
problem. In the original version of this, the stack and heap are set
up after relocation. In the C port, they're set up before, so there's
even more to avoid. To illustrate... here's a CONFIG_RELOCATABLE=n

input: 0x0000000001dfe24d-0x00000000023db865
output: 0x0000000001000000-0x00000000023c98c0
heap: 0x00000000023e0740-0x00000000023e8740
stack: 0x00000000023ec698
chosen: 0x0000000001000000

(stack is just cheating and reporting sp in decompress_kernel)

And a CONFIG_RELOCATABLE=y and "noaslr" boot:

input: 0x000000000108b25e-0x00000000016b3e96
output: 0x0000000000200000-0x00000000016a1db8
heap: 0x00000000016b9600-0x00000000016c1600
stack: 0x00000000016c5558
chosen: 0x0000000000200000

In that case, it's just so far under LOAD_PHYSICAL_START that it's
safe. But if KASLR picks an area overlapping input, heap, or stack
it's hosed. :)


Kees Cook
Chrome OS Security
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