[PATCH] efi/arm64: Update debug prints to reflect other entropy sources

[Ard Biesheuvel]

On Tue, 19 Jan 2021 at 19:26, Mark Brown <broonie at kernel.org> wrote:
>
> Currently the EFI stub prints a diagnostic on boot saying that KASLR will
> be disabled if it is unable to use the EFI RNG protocol to obtain a seed
> for KASLR. When this code was originally added there were no other entropy
> sources which KASLR would be able to obtain entropy from so this was
> accurate but with the addition of support of v8.5-RNG and the SMCCC RNG
> protocol it is now possible for KASLR to obtain entropy even if the EFI
> RNG protocol is unsupported in the system. This has been seen in emulated
> systems with EFI.
>
> Weaken the diagnostic to reflect the fact that KASLR may not be disabled,
> the warning is still useful as other sources may not be available and newer
> versions of both SBBR and EBBR require the RNG protocol and recommend that
> it be able to provide sufficient entropy for seeding KASLR.
>
> Signed-off-by: Mark Brown <broonie at kernel.org>
> ---
>  drivers/firmware/efi/libstub/arm64-stub.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index 22ece1ad68a8..94ade08b1054 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -61,10 +61,10 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>                         status = efi_get_random_bytes(sizeof(phys_seed),
>                                                       (u8 *)&phys_seed);
>                         if (status == EFI_NOT_FOUND) {
> -                               efi_info("EFI_RNG_PROTOCOL unavailable, KASLR will be disabled\n");
> +                               efi_info("EFI_RNG_PROTOCOL unavailable, KASLR may be disabled\n");
>                                 efi_nokaslr = true;
>                         } else if (status != EFI_SUCCESS) {
> -                               efi_err("efi_get_random_bytes() failed (0x%lx), KASLR will be disabled\n",
> +                               efi_err("efi_get_random_bytes() failed (0x%lx), KASLR may be disabled\n",
>                                         status);
>                                 efi_nokaslr = true;
>                         }
> --
> 2.20.1
>

The EFI stub randomizes the physical placement of the kernel as well,
and this is no longer possible by the time we reach early_kaslr_init()
in the kernel proper, so this is not something RNDR et al can make up
for.

So perhaps change this to 'physical placement will not be randomized'
or something along those lines? Or alternatively, just remove the
second part of the sentence - we have better reporting of the KASLR
state now anyway.