[PATCH] arm64: fix address fault during mapping fdt region

[zijun_hu@zoho.com (zijun_hu)]

>From 07b9216ec3494515e7a6c41e0333eb8782427db3 Mon Sep 17 00:00:00 2001
From: zijun_hu <zijun_hu@htc.com>
Date: Mon, 1 Aug 2016 17:04:59 +0800
Subject: [PATCH] arm64: fix address fault during mapping fdt region

fdt_check_header() accesses other fileds of fdt header but
the first 8 bytes such as version; so accessing unmapped
address fault happens if fdt region locates below align
boundary nearly during mapping fdt region, or expressed as
(offset + sizeof(struct fdt_header)) > SWAPPER_BLOCK_SIZE

fdt header size at least is mapped in order to avoid the issue

Signed-off-by: zijun_hu <zijun_hu@htc.com>
---
 arch/arm64/mm/mmu.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 0f85a46..0d72b71 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -744,6 +744,7 @@ void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 	const u64 dt_virt_base = __fix_to_virt(FIX_FDT);
 	int offset;
 	void *dt_virt;
+	int dt_header_map_size;
 
 	/*
 	 * Check whether the physical FDT address is set and meets the minimum
@@ -774,9 +775,18 @@ void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 	offset = dt_phys % SWAPPER_BLOCK_SIZE;
 	dt_virt = (void *)dt_virt_base + offset;
 
+	/*
+	 * fdt_check_header() maybe access any field of fdt header not
+	 * the first 8 bytes only, so map fdt header size at least for
+	 * checking fdt header without address fault more portably
+	 */
+	BUILD_BUG_ON(sizeof(struct fdt_header) > SWAPPER_BLOCK_SIZE);
+	dt_header_map_size = round_up(offset + sizeof(struct fdt_header),
+			SWAPPER_BLOCK_SIZE);
+
 	/* map the first chunk so we can read the size from the header */
 	create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE),
-			dt_virt_base, SWAPPER_BLOCK_SIZE, prot);
+			dt_virt_base, dt_header_map_size, prot);
 
 	if (fdt_check_header(dt_virt) != 0)
 		return NULL;
@@ -785,7 +795,7 @@ void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 	if (*size > MAX_FDT_SIZE)
 		return NULL;
 
-	if (offset + *size > SWAPPER_BLOCK_SIZE)
+	if (offset + *size > dt_header_map_size)
 		create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE), dt_virt_base,
 			       round_up(offset + *size, SWAPPER_BLOCK_SIZE), prot);
 
-- 
1.9.1