Hi Ard,
On Thu, 20 Oct 2022 at 11:40, Ard Biesheuvel ardb@kernel.org wrote:
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.
Can we add this as a comment right above the efi_bs_call()
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
drivers/firmware/efi/libstub/random.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/firmware/efi/libstub/random.c b/drivers/firmware/efi/libstub/random.c index 24aa37535372..183dc5cdb8ed 100644 --- a/drivers/firmware/efi/libstub/random.c +++ b/drivers/firmware/efi/libstub/random.c @@ -75,7 +75,7 @@ efi_status_t efi_random_get_seed(void) if (status != EFI_SUCCESS) return status;
status = efi_bs_call(allocate_pool, EFI_RUNTIME_SERVICES_DATA,
status = efi_bs_call(allocate_pool, EFI_ACPI_RECLAIM_MEMORY, sizeof(*seed) + EFI_RANDOM_SEED_SIZE, (void **)&seed); if (status != EFI_SUCCESS)-- 2.35.1
Reviewed-by: Ilias Apalodimas ilias.apalodimas@linaro.org