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

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.

Reported-by: Zijun Hu <zijun_hu@htc.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 116ad654dd593d18a07d68fd61875dd2de614222..653735a8c58a86248e37593648de4a62f301893c 100644 (file)
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -652,9 +652,9 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
        /*
         * 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)
@@ -682,7 +682,7 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
        create_mapping(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);