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

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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>
This commit is contained in:
Ard Biesheuvel 2020-04-13 15:36:37 +02:00
parent 82046702e2
commit 5d12da9dd6
1 changed files with 5 additions and 27 deletions
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32
drivers/firmware/efi/libstub/arm64-stub.c
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@ -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;