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

From: James Morse
Date: Fri Jul 27 2018 - 05:22:33 EST

Hi Akashi,

On 07/27/2018 09:31 AM, AKASHI Takahiro wrote:
On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote:
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.
Okay, I'll update my terminology!

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).

I think we're speaking at cross purposes. Don't you need:
| buf_size += fdt_prop_len("kaslrâseed", sizeof(u64));

You can't assume the existing DTB had a kaslr-seed property, and the difference may take us over a PAGE_SIZE boundary.

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

(I really don't understand this 'copying code from user-space' that happens with kexec_file_load)

  if (not found kaslr-seed in 1st kernel's dtb)
     don't care; go ahead

Don' t bother. As you say in the commit-message its harmless if the new kernel doesn't support it.
Always having this would let you use kexec_file_load as a bootloader that can get the crng to
provide decent entropy even if the platform bootloader can't.

     if (current kaslr-seed != 0)

Don't bother. If this happens its a bug in another part of the kernel that doesn't affect this one. We aren't second-guessing the file-system when we read the kernel-fd, lets keep this simple.

     if (crng_ready()) ; FIXME, it's a local macro
        set new kaslr-seed
error? Something like pr_warn_once().

I thought the kaslr-seed was added to the entropy pool, but now I look again I see its a separate EFI table. So the new kernel will add the same entropy ... that doesn't sound clever. (I can't see where its zero'd or re-initialised)