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

From: AKASHI Takahiro
Date: Fri Jul 27 2018 - 04:29:35 EST

On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote:
> 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.

Yes, but I think that I and Mark has agreed that "kaslr" meant
"virtual" randomisation, not including "physical" randomisation.

> 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?

No, I don't think so.
All the data to be loaded are temporarily saved in kexec buffers,
which will eventually be copied to target locations in machine_kexec
(arm64_relocate_new_kernel, which, unlike its name, will handle
not only kernel but also other data as well).

> > + /* 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
> unconditionally.
> 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.

OK. I would try to follow the same way as Bhupesh's userspace patch
does for kaslr-seed:

if (not found kaslr-seed in 1st kernel's dtb)
don't care; go ahead
if (current kaslr-seed != 0)
if (crng_ready()) ; FIXME, it's a local macro
set new kaslr-seed

> > + 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.

OK. (but in this file for now as I mentioned in my previous reply)

-Takahiro AKASHI

> Thanks,
> James