Commit 5d12da9d authored by Ard Biesheuvel's avatar Ard Biesheuvel
Browse files

efi/libstub/arm64: Simplify randomized loading of kernel image 



The KASLR code path in the arm64 version of the EFI stub incorporates
some overly complicated logic to randomly allocate a region of the right
alignment: there is no need to randomize the placement of the kernel
modulo 2 MiB separately from the placement of the 2 MiB aligned allocation
itself - we can simply follow the same logic used by the non-randomized
placement, which is to allocate at the correct alignment, and only take
TEXT_OFFSET into account if it is not a round multiple of the alignment.

Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
parent 82046702

drivers/firmware/efi/libstub/arm64-stub.c

+5 −27
Original line number Diff line number Diff line
@@ -52,7 +52,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr, 
{

	efi_status_t status;

	unsigned long kernel_size, kernel_memsize = 0;

	u64 phys_seed = 0;

	u32 phys_seed = 0;



	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {

		if (!nokaslr()) {
@@ -74,36 +74,15 @@ efi_status_t handle_kernel_image(unsigned long *image_addr, 


	kernel_size = _edata - _text;

	kernel_memsize = kernel_size + (_end - _edata);

	*reserve_size = kernel_memsize + TEXT_OFFSET % min_kimg_align;



	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && phys_seed != 0) {

		/*

		 * Produce a displacement in the interval [0, MIN_KIMG_ALIGN)

		 * that doesn't violate this kernel's de-facto alignment

		 * constraints.

		 */

		u32 mask = (MIN_KIMG_ALIGN - 1) & ~(EFI_KIMG_ALIGN - 1);

		u32 offset = (phys_seed >> 32) & mask;



		/*

		 * With CONFIG_RANDOMIZE_TEXT_OFFSET=y, TEXT_OFFSET may not

		 * be a multiple of EFI_KIMG_ALIGN, and we must ensure that

		 * we preserve the misalignment of 'offset' relative to

		 * EFI_KIMG_ALIGN so that statically allocated objects whose

		 * alignment exceeds PAGE_SIZE appear correctly aligned in

		 * memory.

		 */

		offset |= TEXT_OFFSET % EFI_KIMG_ALIGN;



		/*

		 * If KASLR is enabled, and we have some randomness available,

		 * locate the kernel at a randomized offset in physical memory.

		 */

		*reserve_size = kernel_memsize + offset;

		status = efi_random_alloc(*reserve_size,

					  MIN_KIMG_ALIGN, reserve_addr,

					  (u32)phys_seed);



		*image_addr = *reserve_addr + offset;

		status = efi_random_alloc(*reserve_size, min_kimg_align,

					  reserve_addr, phys_seed);

	} else {

		status = EFI_OUT_OF_RESOURCES;

	}
@@ -119,7 +98,6 @@ efi_status_t handle_kernel_image(unsigned long *image_addr, 
			return EFI_SUCCESS;

		}



		*reserve_size = kernel_memsize + TEXT_OFFSET % min_kimg_align;

		status = efi_low_alloc(*reserve_size,

				       min_kimg_align, reserve_addr);
@@ -128,9 +106,9 @@ efi_status_t handle_kernel_image(unsigned long *image_addr, 
			*reserve_size = 0;

			return status;

		}

		*image_addr = *reserve_addr + TEXT_OFFSET % min_kimg_align;

	}



	*image_addr = *reserve_addr + TEXT_OFFSET % min_kimg_align;

	memcpy((void *)*image_addr, _text, kernel_size);



	return EFI_SUCCESS;