From a6cfe03c34bad8c7f51aba49a73403e348c51d1f Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Tue, 3 May 2022 14:29:39 +0200 Subject: [PATCH] efi: stub: prefer mirrored memory for randomized allocations 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 Reviewed-by: Kefeng Wang --- 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 724155b9e10dc..715f374791542 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; } + /* consider only mirrored slots for randomization if any exist */ + if (total_mirrored_slots > 0) + total_slots = total_mirrored_slots; + /* 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;