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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2022/5/5 21:43, Kefeng Wang 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@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.

Please ignore this, the kernel only check whether the memory below 4G are

all mirrored memory or not.

>
> 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;