[PATCH] char: mem: keep arch range checks overflow-safe

[Yousef Alhouseen]

The generic /dev/mem physical range check now avoids validating
addr + size directly, but ARM and SH provide their own
valid_phys_addr_range() implementations with the same wrapped-addition
pattern.

Use subtraction-based upper-bound checks in those overrides as well. Also
make the ARM mmap pfn check avoid overflowing the pfn plus page-count
expression.

Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Yousef Alhouseen <alhouseenyousef@xxxxxxxxx>
---
 arch/arm/mm/mmap.c | 9 +++++++--
 arch/sh/mm/mmap.c  | 4 +++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 3dbb383c2..7467ce8cd 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -150,9 +150,11 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
  */
 int valid_phys_addr_range(phys_addr_t addr, size_t size)
 {
+	phys_addr_t end = __pa(high_memory - 1) + 1;
+
 	if (addr < PHYS_OFFSET)
 		return 0;
-	if (addr + size > __pa(high_memory - 1) + 1)
+	if (addr > end || size > end - addr)
 		return 0;
 
 	return 1;
@@ -163,5 +165,8 @@ int valid_phys_addr_range(phys_addr_t addr, size_t size)
  */
 int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
 {
-	return (pfn + (size >> PAGE_SHIFT)) <= (1 + (PHYS_MASK >> PAGE_SHIFT));
+	unsigned long max_pfn = 1 + (PHYS_MASK >> PAGE_SHIFT);
+	unsigned long pages = size >> PAGE_SHIFT;
+
+	return pfn <= max_pfn && pages <= max_pfn - pfn;
 }
diff --git a/arch/sh/mm/mmap.c b/arch/sh/mm/mmap.c
index c442734d9..ba7aade46 100644
--- a/arch/sh/mm/mmap.c
+++ b/arch/sh/mm/mmap.c
@@ -170,9 +170,11 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
  */
 int valid_phys_addr_range(phys_addr_t addr, size_t count)
 {
+	phys_addr_t end = __pa(high_memory);
+
 	if (addr < __MEMORY_START)
 		return 0;
-	if (addr + count > __pa(high_memory))
+	if (addr > end || count > end - addr)
 		return 0;
 
 	return 1;
-- 
2.54.0