[PATCH] efi: stub: prefer mirrored memory for randomized allocations

[Kefeng Wang]

On 2022/5/3 23:21, Ard Biesheuvel wrote:
> If the system exposes memory regions with the EFI_MORE_RELIABLE
> attribute, it is implied that it is intended to be used for allocations
> that are relatively important, such as the kernel's static image.
>
> Since efi_random_alloc() is mostly (only) used for allocating space for
> the kernel image, let's update it to take this into account, and
> disregard all memory without the EFI_MORE_RELIABLE attribute if there is
> sufficient memory available that does have this attribute.
>
> Note that this change only affects booting with randomization enabled.
> In other cases, the EFI stub runs the kernel image in place unless its
> placement is unsuitable for some reason (i.e., misaligned, or its BSS
> overlaps with another allocation), and it is left to the bootloader to
> ensure that the kernel was loaded into EFI_MORE_RELIABLE memory if this
> is desired.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>   drivers/firmware/efi/libstub/randomalloc.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/randomalloc.c b/drivers/firmware/efi/libstub/randomalloc.c
> index 724155b9e10d..07a762910312 100644
> --- a/drivers/firmware/efi/libstub/randomalloc.c
> +++ b/drivers/firmware/efi/libstub/randomalloc.c
> @@ -56,6 +56,7 @@ efi_status_t efi_random_alloc(unsigned long size,
>   			      unsigned long random_seed)
>   {
>   	unsigned long map_size, desc_size, total_slots = 0, target_slot;
> +	unsigned long total_mirrored_slots = 0;
>   	unsigned long buff_size;
>   	efi_status_t status;
>   	efi_memory_desc_t *memory_map;
> @@ -86,8 +87,14 @@ efi_status_t efi_random_alloc(unsigned long size,
>   		slots = get_entry_num_slots(md, size, ilog2(align));
>   		MD_NUM_SLOTS(md) = slots;
>   		total_slots += slots;
> +		if (md->attribute & EFI_MEMORY_MORE_RELIABLE)
> +			total_mirrored_slots += slots;
>   	}
>   
> +	/* only consider mirrored slots for randomization if any exist */
> +	if (total_mirrored_slots > 0)
> +		total_slots = total_mirrored_slots;
> +

The kernel will check 4G lower limit to enable kernelcore=mirror feature.

Do we need some fallback mechanism in case of small mirror slots which

leads to fail allocation for Image?


>   	/* find a random number between 0 and total_slots */
>   	target_slot = (total_slots * (u64)(random_seed & U32_MAX)) >> 32;
>   
> @@ -107,6 +114,10 @@ efi_status_t efi_random_alloc(unsigned long size,
>   		efi_physical_addr_t target;
>   		unsigned long pages;
>   
> +		if (total_mirrored_slots > 0 &&
> +		    !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
> +			continue;
> +
>   		if (target_slot >= MD_NUM_SLOTS(md)) {
>   			target_slot -= MD_NUM_SLOTS(md);
>   			continue;