Re: [PATCH v12 16/16] arm64: kexec_file: add kaslr support

From: James Morse
Date: Thu Jul 26 2018 - 09:40:56 EST

Hi Akashi,

On 24/07/18 07:57, AKASHI Takahiro wrote:
> Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual
> address randomization, at secondary kernel boot.

Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The kernel
physical placement when booted via the EFIstub, the kernel-text VAs and the
location of memory in the linear-map region. Adding the kaslr-seed only does the
last two.

This means the physical placement of the new kernel is predictable from
/proc/iomem ... but this also tells you the physical placement of the current
kernel, so I don't think this is a problem.

> We always do this as it will have no harm on kaslr-incapable kernel.

> We don't have any "switch" to turn off this feature directly, but still
> can suppress it by passing "nokaslr" as a kernel boot argument.

> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> index 7356da5a53d5..47a4fbd0dc34 100644
> --- a/arch/arm64/kernel/machine_kexec_file.c
> +++ b/arch/arm64/kernel/machine_kexec_file.c
> @@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image,

Don't you need to reserve some space in the area you vmalloc()d for the DT?

> + /* add kaslr-seed */
> + get_random_bytes(&value, sizeof(value));

What happens if the crng isn't ready?

It looks like this will print a warning that these random-bytes aren't really up
to standard, but the new kernel doesn't know this happened.

crng_ready() isn't exposed, all we could do now is
wait_for_random_bytes(), but that may wait forever because we do this

I'd prefer to leave this feature until we can check crng_ready(), and skip
adding a dodgy-seed if its not-ready. This avoids polluting the next-kernel's
entropy pool.

> + ret = fdt_setprop(buf, nodeoffset, "kaslr-seed", &value, sizeof(value));

Nit: It would be nice if this string were in a header file somewhere, to void
future refactoring typos.