Patch "efi/fdt: Avoid FDT manipulation after ExitBootServices()" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

efi/fdt: Avoid FDT manipulation after ExitBootServices()

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
efi-fdt-avoid-fdt-manipulation-after-exitbootservices.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ard Biesheuvel <[email protected]>
Date: Wed, 1 Feb 2017 17:45:02 +0000
Subject: efi/fdt: Avoid FDT manipulation after ExitBootServices()

From: Ard Biesheuvel <[email protected]>

commit c8f325a59cfc718d13a50fbc746ed9b415c25e92 upstream.

Some AArch64 UEFI implementations disable the MMU in ExitBootServices(), after which unaligned accesses to RAM are no longer supported.

Commit:

abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")

fixed an issue in the memory map handling of the stub FDT code, but inadvertently created an issue with such firmware, by moving some of the FDT manipulation to after the invocation of ExitBootServices().

Given that the stub's libfdt implementation uses the ordinary, accelerated string functions, which rely on hardware handling of unaligned accesses, manipulating the FDT with the MMU off may result in alignment faults.

So fix the situation by moving the update_fdt_memmap() call into the callback function invoked by efi_exit_boot_services() right before it calls the ExitBootServices() UEFI service (which is arguably a better place for it anyway)

Note that disabling the MMU in ExitBootServices() is not compliant with the UEFI spec, and carries great risk due to the fact that switching from cached to uncached memory accesses halfway through compiler generated code (i.e., involving a stack) can never be done in a way that is architecturally safe.

---
 drivers/firmware/efi/libstub/fdt.c |   14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

@@ -187,6 +187,7 @@ static efi_status_t update_fdt_memmap(vo
 struct exit_boot_struct {
 	efi_memory_desc_t *runtime_map;
 	int *runtime_entry_count;
+	void *new_fdt_addr;
 };
 
 static efi_status_t exit_boot_func(efi_system_table_t *sys_table_arg,

@@ -202,7 +203,7 @@ static efi_status_t exit_boot_func(efi_s
 	efi_get_virtmap(*map->map, *map->map_size, *map->desc_size,
 			p->runtime_map, p->runtime_entry_count);
 
-	return EFI_SUCCESS;
+	return update_fdt_memmap(p->new_fdt_addr, map);
 }
 
 /*

@@ -300,22 +301,13 @@ efi_status_t allocate_new_fdt_and_exit_b
 
 	priv.runtime_map = runtime_map;
 	priv.runtime_entry_count = &runtime_entry_count;
+	priv.new_fdt_addr = (void *)*new_fdt_addr;
 	status = efi_exit_boot_services(sys_table, handle, &map, &priv,
 					exit_boot_func);
 
 	if (status == EFI_SUCCESS) {
 		efi_set_virtual_address_map_t *svam;
 
-		status = update_fdt_memmap((void *)*new_fdt_addr, &map);
-		if (status != EFI_SUCCESS) {
-			/*
-			 * The kernel won't get far without the memory map, but
-			 * may still be able to print something meaningful so
-			 * return success here.
-			 */
-			return EFI_SUCCESS;
-		}
-
 		/* Install the new virtual address map */
 		svam = sys_table->runtime->set_virtual_address_map;
 		status = svam(runtime_entry_count * desc_size, desc_size,

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/crypto-arm64-aes-blk-honour-iv_out-requirement-in-cbc-and-ctr-modes.patch queue-4.9/efi-fdt-avoid-fdt-manipulation-after-exitbootservices.patch queue-4.9/x86-efi-always-map-the-first-physical-page-into-the-efi-pagetables.patch