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

[Ard Biesheuvel <ardb@kernel.org>]

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;
+
 	/* 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;
-- 
2.30.2