arm64: efi: kaslr: Fix occasional random alloc (and boot) failure

arm64: efi: kaslr: Fix occasional random alloc (and boot) failure

The EFI stub random allocator used for kaslr on arm64 has a subtle
bug. In function get_entry_num_slots() which counts the number of
possible allocation "slots" for the image in a given chunk of free
EFI memory, "last_slot" can become negative if the chunk is smaller
than the requested allocation size.

The test "if (first_slot > last_slot)" doesn't catch it because
both first_slot and last_slot are unsigned.

I chose not to make them signed to avoid problems if this is ever
used on architectures where there are meaningful addresses with the
top bit set. Instead, fix it with an additional test against the
allocation size.

This can cause a boot failure in addition to a loss of randomisation
due to another bug in the arm64 stub fixed separately.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Fixes: 2ddbfc81eac8 ("efi: stub: add implementation of efi_random_alloc()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>