[PATCH v4] of: fdt: fix possible overflow during parsing of fdt

[Abdelrahman Youssef <abdelrahmanyossef12@gmail.com>]

While fuzzing, the name marked by FDT_BEGIN_NODE sometimes extends beyond
the struct block area, causing a heap-overflow.

Since `maxlen` is an unsigned integer representing the length of name,
It can be negative, so it overflows to large numbers, Causing strnlen()
to overflow.

So we can just change the type of maxlen to signed and check if it's a
non-positive value, because name has a minimum length of 1 byte ('\0').

Also in strnlen() we shouldn't check for bytes exceeding maxlen, so we can remove
+ 1 in strnlen(). We also change if (len > maxlen) to >= to count for the null
terminator.

Signed-off-by: Abdelrahman Youssef <abdelrahmanyossef12@gmail.com>

---
v3 -> v4:
  - replace maxlen < 0 to maxlen <= 0 (Sascha)
  - remove + 1 in strnlen() (Sascha)
v2 -> v3
  - changed formatting
v1 -> v2
  - the overflow was due to integer overflow not out-of-bounds (Ahmad)
---
 drivers/of/fdt.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index 2c3ea31394..75af1844f3 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -176,7 +176,7 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
 	void *dt_strings;
 	struct fdt_header f;
 	int ret;
-	unsigned int maxlen;
+	int maxlen;
 	const struct fdt_header *fdt = infdt;
 
 	ret = fdt_parse_header(infdt, size, &f);
@@ -210,8 +210,13 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
 			maxlen = (unsigned long)fdt + f.off_dt_struct +
 				f.size_dt_struct - (unsigned long)name;
 
-			len = strnlen(name, maxlen + 1);
-			if (len > maxlen) {
+			if (maxlen <= 0) {
+				ret = -ESPIPE;
+				goto err;
+			}
+
+			len = strnlen(name, maxlen);
+			if (len >= maxlen) {
 				ret = -ESPIPE;
 				goto err;
 			}
-- 
2.43.0