efi: random: Use 'ACPI reclaim' memory for random seed

EFI runtime services data is guaranteed to be preserved by the OS,
making it a suitable candidate for the EFI random seed table, which may
be passed to kexec kernels as well (after refreshing the seed), and so
we need to ensure that the memory is preserved without support from the
OS itself.

However, runtime services data is intended for allocations that are
relevant to the implementations of the runtime services themselves, and
so they are unmapped from the kernel linear map, and mapped into the EFI
page tables that are active while runtime service invocations are in
progress. None of this is needed for the RNG seed.

So let's switch to EFI 'ACPI reclaim' memory: in spite of the name,
there is nothing exclusively ACPI about it, it is simply a type of
allocation that carries firmware provided data which may or may not be
relevant to the OS, and it is left up to the OS to decide whether to
reclaim it after having consumed its contents.

Given that in Linux, we never reclaim these allocations, it is a good
choice for the EFI RNG seed, as the allocation is guaranteed to survive
kexec reboots.

One additional reason for changing this now is to align it with the
upcoming recommendation for EFI bootloader provided RNG seeds, which
must not use EFI runtime services code/data allocations.

Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

drivers/firmware/efi/libstub/random.c

@@ -75,7 +75,12 @@ efi_status_t efi_random_get_seed(void)
 	if (status != EFI_SUCCESS)
 		return status;
 
-	status = efi_bs_call(allocate_pool, EFI_RUNTIME_SERVICES_DATA,
+	/*
+	 * Use EFI_ACPI_RECLAIM_MEMORY here so that it is guaranteed that the
+	 * allocation will survive a kexec reboot (although we refresh the seed
+	 * beforehand)
+	 */
+	status = efi_bs_call(allocate_pool, EFI_ACPI_RECLAIM_MEMORY,
 			     sizeof(*seed) + EFI_RANDOM_SEED_SIZE,
 			     (void **)&seed);
 	if (status != EFI_SUCCESS)