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

[Ard Biesheuvel]

On 27 July 2018 at 11:22, James Morse <james.morse@xxxxxxx> wrote:
> 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
> 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)
>

We do have a hook for that: grep for update_efi_random_seed()