On 2022/5/6 0:12, Ard Biesheuvel wrote:
On Thu, 5 May 2022 at 15:43, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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@xxxxxxxxxx>
/* 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;
.
