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

[Kefeng Wang]

On 2022/5/6 0:12, Ard Biesheuvel wrote:
> On Thu, 5 May 2022 at 15:43, Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
>>
>> 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.
>>
> Why? I mean, why is 4G a magic number also on arm64?
Please ignore this, replied in the previous email.
>
>> Do we need some fallback mechanism in case of small mirror slots which
>>
>> leads to fail allocation for Image?
>>
> This code only counts slots that are large enough to hold the Image so
> this can never happen. If total_mirrored_slots > 0, there is at least
> one possible placement of the kernel where it falls entirely inside a
> EFI_MORE_RELIABLE region.

I see,  slots = get_entry_num_slots(md, *size*, ilog2(align));

Thanks.

Reviewed-by: Kefeng Wang <wangkefeng.wang at huawei.com>

>
>>>        /* 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;
> .