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

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("kaslr?seed", 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
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:
http://lists.infradead.org/pipermail/kexec/2018-April/020564.html


(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.


  else
     if (current kaslr-seed != 0)
        error


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
        get_random_bytes(non-blocking)
        set new kaslr-seed
     else
        error

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)