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: Ard Biesheuvel <ardb@kernel.org>

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index cfd535c..6fc3bd9 100644 (file)
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -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;