net: don't unconditionally copy_from_user a struct ifreq for socket ioctls

A common implementation of isatty(3) involves calling a ioctl passing
a dummy struct argument and checking whether the syscall failed --
bionic and glibc use TCGETS (passing a struct termios), and musl uses
TIOCGWINSZ (passing a struct winsize). If the FD is a socket, we will
copy sizeof(struct ifreq) bytes of data from the argument and return
-EFAULT if that fails. The result is that the isatty implementations
may return a non-POSIX-compliant value in errno in the case where part
of the dummy struct argument is inaccessible, as both struct termios
and struct winsize are smaller than struct ifreq (at least on arm64).

Although there is usually enough stack space following the argument
on the stack that this did not present a practical problem up to now,
with MTE stack instrumentation it's more likely for the copy to fail,
as the memory following the struct may have a different tag.

Fix the problem by adding an early check for whether the ioctl is a
valid socket ioctl, and return -ENOTTY if it isn't.

Fixes: 44c02a2c ("dev_ioctl(): move copyin/copyout to callers")
Link: https://linux-review.googlesource.com/id/I869da6cf6daabc3e4b7b82ac979683ba05e27d4dSigned-off-by: Peter Collingbourne <pcc@google.com>
Cc: <stable@vger.kernel.org> # 4.19
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -4012,6 +4012,10 @@ int netdev_rx_handler_register(struct net_device *dev,
 void netdev_rx_handler_unregister(struct net_device *dev);
 
 bool dev_valid_name(const char *name);
+static inline bool is_socket_ioctl_cmd(unsigned int cmd)
+{
+	return _IOC_TYPE(cmd) == SOCK_IOC_TYPE;
+}
 int dev_ioctl(struct net *net, unsigned int cmd, struct ifreq *ifr,
 		bool *need_copyout);
 int dev_ifconf(struct net *net, struct ifconf *, int);

net/socket.c

@@ -1109,7 +1109,7 @@ static long sock_do_ioctl(struct net *net, struct socket *sock,
 		rtnl_unlock();
 		if (!err && copy_to_user(argp, &ifc, sizeof(struct ifconf)))
 			err = -EFAULT;
-	} else {
+	} else if (is_socket_ioctl_cmd(cmd)) {
 		struct ifreq ifr;
 		bool need_copyout;
 		if (copy_from_user(&ifr, argp, sizeof(struct ifreq)))
@@ -1118,6 +1118,8 @@ static long sock_do_ioctl(struct net *net, struct socket *sock,
 		if (!err && need_copyout)
 			if (copy_to_user(argp, &ifr, sizeof(struct ifreq)))
 				return -EFAULT;
+	} else {
+		err = -ENOTTY;
 	}
 	return err;
 }
@@ -3306,6 +3308,8 @@ static int compat_ifr_data_ioctl(struct net *net, unsigned int cmd,
 	struct ifreq ifreq;
 	u32 data32;
 
+	if (!is_socket_ioctl_cmd(cmd))
+		return -ENOTTY;
 	if (copy_from_user(ifreq.ifr_name, u_ifreq32->ifr_name, IFNAMSIZ))
 		return -EFAULT;
 	if (get_user(data32, &u_ifreq32->ifr_data))