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

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 6 May 2022 at 03:43, Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>
>
> On 2022/5/6 0:12, Ard Biesheuvel wrote:
> > On Thu, 5 May 2022 at 15:43, Kefeng Wang <wangkefeng.wang@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@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@huawei.com>
>

Thank you. I have queued this up now.