efi: stub: add implementation of efi_random_alloc()
This implements efi_random_alloc(), which allocates a chunk of memory of a certain size at a certain alignment, and uses the random_seed argument it receives to randomize the address of the allocation. This is implemented by iterating over the UEFI memory map, counting the number of suitable slots (aligned offsets) within each region, and picking a random number between 0 and 'number of slots - 1' to select the slot, This should guarantee that each possible offset is chosen equally likely. Suggested-by: NKees Cook <keescook@chromium.org> Reviewed-by: NMatt Fleming <matt@codeblueprint.co.uk> Reviewed-by: NKees Cook <keescook@chromium.org> Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
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