Patch "arm64: mm: avoid fdt_check_header() before the FDT is fully mapped" has been added to the 4.7-stable tree

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

arm64: mm: avoid fdt_check_header() before the FDT is fully mapped

to the 4.7-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:
arm64-mm-avoid-fdt_check_header-before-the-fdt-is-fully-mapped.patch
and it can be found in the queue-4.7 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: Mon, 1 Aug 2016 13:29:31 +0200
Subject: arm64: mm: avoid fdt_check_header() before the FDT is fully mapped

From: Ard Biesheuvel <[email protected]>

commit 04a848106193b134741672f7e4e444b50c70b631 upstream.

As reported by Zijun, the fdt_check_header() call in __fixmap_remap_fdt() is not safe since it is not guaranteed that the FDT header is mapped completely. Due to the minimum alignment of 8 bytes, the only fields we can assume to be mapped are 'magic' and 'totalsize'.

Since the OF layer is in charge of validating the FDT image, and we are only interested in making reasonably sure that the size field contains a meaningful value, replace the fdt_check_header() call with an explicit comparison of the magic field's value against the expected value.

---
 arch/arm64/mm/mmu.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

@@ -748,9 +748,9 @@ void *__init __fixmap_remap_fdt(phys_add
 	/*
 	 * Check whether the physical FDT address is set and meets the minimum
 	 * alignment requirement. Since we are relying on MIN_FDT_ALIGN to be
-	 * at least 8 bytes so that we can always access the size field of the
-	 * FDT header after mapping the first chunk, double check here if that
-	 * is indeed the case.
+	 * at least 8 bytes so that we can always access the magic and size
+	 * fields of the FDT header after mapping the first chunk, double check
+	 * here if that is indeed the case.
 	 */
 	BUILD_BUG_ON(MIN_FDT_ALIGN < 8);
 	if (!dt_phys || dt_phys % MIN_FDT_ALIGN)

@@ -778,7 +778,7 @@ void *__init __fixmap_remap_fdt(phys_add
 	create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE),
 			dt_virt_base, SWAPPER_BLOCK_SIZE, prot);
 
-	if (fdt_check_header(dt_virt) != 0)
+	if (fdt_magic(dt_virt) != FDT_MAGIC)
 		return NULL;
 
 	*size = fdt_totalsize(dt_virt);

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

queue-4.7/arm64-only-select-arm64_module_plts-if-modules-y.patch queue-4.7/arm64-vmlinux.lds-make-__rela_offset-and-__dynsym_offset-absolute.patch queue-4.7/arm64-mm-avoid-fdt_check_header-before-the-fdt-is-fully-mapped.patch