[RFC PATCH] ARM: decompressor: implement autonomous KASLR offset calculation

[Ard Biesheuvel]

On 15 August 2017 at 21:29, Kees Cook <keescook at chromium.org> wrote:
> On Tue, Aug 15, 2017 at 1:15 PM, Ard Biesheuvel
> <ard.biesheuvel at linaro.org> wrote:
>> This enables KASLR for environments that are not KASLR-aware, or only
>> to a limited extent. The decompressor collects information about the
>> placement of the zImage, DTB and initrd, and parses the /memory DT
>> node and the /memreserve/s and /reserved-memory node, and combines this
>> information to select a suitable KASLR offset, and proceeds to decompress
>> the kernel at this offset in physical memory. It then invoked the kernel
>> proper while passing on this information, so that it can be taken into
>> account to create the virtual mapping.
>>
>> This code shuffles some registers together to create a poor man's seed,
>> which will be superseded by the value of /chosen/kaslr-seed if present.
>>
[...]
>
> Without kaslr-seed, perhaps also include build-time entropy (as I did
> in x86's decompressor with build_str):
>
> static unsigned long get_boot_seed(void)
> {
>         unsigned long hash = 0;
>
>         hash = rotate_xor(hash, build_str, sizeof(build_str));
>         hash = rotate_xor(hash, boot_params, sizeof(*boot_params));
>
>         return hash;
> }
>

Shouldn't we use something with better diffusion than rotate and xor?
If we can afford to decompress ~10 MB worth of vmlinux, I'm sure we
can afford to crc() the device tree blob.

> You're effectively starting with hash == r3. This could be further
> enhanced with a __latent_entropy string when that plugin is enabled:
>
> static u8 compile_time_entropy[32] __latent_entropy;
>
> ...
>
> seed = rotate_xor(seed, compile_time_entropy, sizeof(compile_time_entropy));
>
> And toss in the fdt too? (I have no idea if this is the correct way to
> do this....)
>
> seed = rotate_xor(seed, fdt, fdt_totalsize(fdt));
>

As mentioned by several people, this is a nice way to add
/chosen/kaslr-seed to the mix if it is there, and still have some
pseudo=entropy otherwise, especially given the fact (as Daniel figured
out) that Android puts device serials and some boot timing related
variables on the kernel command line.