mtdchar: fix integer overflow in read/write ioctls

The "req.start" and "req.len" variables are u64 values that come from the
user at the start of the function.  We mask away the high 32 bits of
"req.len" so that's capped at U32_MAX but the "req.start" variable can go
up to U64_MAX which means that the addition can still integer overflow.

Use check_add_overflow() to fix this bug.

Fixes: 095bb6e ("mtdchar: add MEMREAD ioctl")
Fixes: 6420ac0 ("mtdchar: prevent unbounded allocation in MEMWRITE ioctl")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

Author: Dan Carpenter

drivers/mtd/mtdchar.c

@@ -599,6 +599,7 @@ mtdchar_write_ioctl(struct mtd_info *mtd, struct mtd_write_req __user *argp)
 	uint8_t *datbuf = NULL, *oobbuf = NULL;
 	size_t datbuf_len, oobbuf_len;
 	int ret = 0;
+	u64 end;
 
 	if (copy_from_user(&req, argp, sizeof(req)))
 		return -EFAULT;
@@ -618,7 +619,7 @@ mtdchar_write_ioctl(struct mtd_info *mtd, struct mtd_write_req __user *argp)
 	req.len &= 0xffffffff;
 	req.ooblen &= 0xffffffff;
 
-	if (req.start + req.len > mtd->size)
+	if (check_add_overflow(req.start, req.len, &end) || end > mtd->size)
 		return -EINVAL;
 
 	datbuf_len = min_t(size_t, req.len, mtd->erasesize);
@@ -698,6 +699,7 @@ mtdchar_read_ioctl(struct mtd_info *mtd, struct mtd_read_req __user *argp)
 	size_t datbuf_len, oobbuf_len;
 	size_t orig_len, orig_ooblen;
 	int ret = 0;
+	u64 end;
 
 	if (copy_from_user(&req, argp, sizeof(req)))
 		return -EFAULT;
@@ -724,7 +726,7 @@ mtdchar_read_ioctl(struct mtd_info *mtd, struct mtd_read_req __user *argp)
 	req.len &= 0xffffffff;
 	req.ooblen &= 0xffffffff;
 
-	if (req.start + req.len > mtd->size) {
+	if (check_add_overflow(req.start, req.len, &end) || end > mtd->size) {
 		ret = -EINVAL;
 		goto out;
 	}