[PATCH] tun: make sure interface usage can not overflow

[Kees Cook <keescook@chromium.org>]

This makes the size argument a const, since it is always populated by
the caller. Additionally double-checks to make sure the copy_from_user
can never overflow, keeping CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy:

   In function 'copy_from_user',
       inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1871:7:
       ... copy_from_user() buffer size is not provably correct

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/net/tun.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index acaaf6784179..a1f317cba206 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1855,7 +1855,7 @@ unlock:
 }
 
 static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
-			    unsigned long arg, int ifreq_len)
+			    unsigned long arg, const size_t ifreq_len)
 {
 	struct tun_file *tfile = file->private_data;
 	struct tun_struct *tun;
@@ -1869,6 +1869,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	int ret;
 
 	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == 0x89) {
+		BUG_ON(ifreq_len > sizeof(ifr));
 		if (copy_from_user(&ifr, argp, ifreq_len))
 			return -EFAULT;
 	} else {
-- 
1.9.1


-- 
Kees Cook
Chrome OS Security