[PATCH 1/2] arm64: efi: kaslr: Fix occasional random alloc (and boot) failure

[Ard Biesheuvel]

On Tue, 20 Jul 2021 at 13:14, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
>
> 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 at kernel.crashing.org>
> Fixes: 2ddbfc81eac8 (efi: stub: add implementation of efi_random_alloc())

Thanks for fixing this.

Acked-by: Ard Biesheuvel <ardb at kernel.org>


> ---
>
>  drivers/firmware/efi/libstub/randomalloc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/randomalloc.c b/drivers/firmware/efi/libstub/randomalloc.c
> index a408df474d83..724155b9e10d 100644
> --- a/drivers/firmware/efi/libstub/randomalloc.c
> +++ b/drivers/firmware/efi/libstub/randomalloc.c
> @@ -30,6 +30,8 @@ static unsigned long get_entry_num_slots(efi_memory_desc_t *md,
>
>         region_end = min(md->phys_addr + md->num_pages * EFI_PAGE_SIZE - 1,
>                          (u64)ULONG_MAX);
> +       if (region_end < size)
> +               return 0;
>
>         first_slot = round_up(md->phys_addr, align);
>         last_slot = round_down(region_end - size + 1, align);
>
>