efi: libstub: Drop randomization of runtime memory map

Randomizing the UEFI runtime memory map requires the use of the
SetVirtualAddressMap() EFI boot service, which we prefer to avoid. So
let's drop randomization, which was already problematic in combination
with hibernation, which means that distro kernels never enabled it in
the first place.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: ardbiesheuvel

drivers/firmware/efi/libstub/efi-stub.c

@@ -35,15 +35,6 @@
  * as well to minimize the code churn.
  */
 #define EFI_RT_VIRTUAL_BASE	SZ_512M
-#define EFI_RT_VIRTUAL_SIZE	SZ_512M
-
-#ifdef CONFIG_ARM64
-# define EFI_RT_VIRTUAL_LIMIT	DEFAULT_MAP_WINDOW_64
-#elif defined(CONFIG_RISCV) || defined(CONFIG_LOONGARCH)
-# define EFI_RT_VIRTUAL_LIMIT	TASK_SIZE_MIN
-#else /* Only if TASK_SIZE is a constant */
-# define EFI_RT_VIRTUAL_LIMIT	TASK_SIZE
-#endif
 
 /*
  * Some architectures map the EFI regions into the kernel's linear map using a
@@ -230,26 +221,6 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
 	efi_novamap |= !(get_supported_rt_services() &
 			 EFI_RT_SUPPORTED_SET_VIRTUAL_ADDRESS_MAP);
 
-	/* hibernation expects the runtime regions to stay in the same place */
-	if (!IS_ENABLED(CONFIG_HIBERNATION) && !efi_nokaslr && !flat_va_mapping) {
-		/*
-		 * Randomize the base of the UEFI runtime services region.
-		 * Preserve the 2 MB alignment of the region by taking a
-		 * shift of 21 bit positions into account when scaling
-		 * the headroom value using a 32-bit random value.
-		 */
-		static const u64 headroom = EFI_RT_VIRTUAL_LIMIT -
-					    EFI_RT_VIRTUAL_BASE -
-					    EFI_RT_VIRTUAL_SIZE;
-		u32 rnd;
-
-		status = efi_get_random_bytes(sizeof(rnd), (u8 *)&rnd);
-		if (status == EFI_SUCCESS) {
-			virtmap_base = EFI_RT_VIRTUAL_BASE +
-				       (((headroom >> 21) * rnd) >> (32 - 21));
-		}
-	}
-
 	install_memreserve_table();
 
 	status = efi_boot_kernel(handle, image, image_addr, cmdline_ptr);