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Subject[PATCH 22/33] efi/libstub/arm64: Simplify randomized loading of kernel image
Date
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>
---
drivers/firmware/efi/libstub/arm64-stub.c | 32 ++++-------------------
1 file changed, 5 insertions(+), 27 deletions(-)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index cfd535c13242..6fc3bd9a56db 100644
--- 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;
--
2.17.1
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