net: fix info leak in compat dev_ifconf()

commit 43da5f2e upstream.

The implementation of dev_ifconf() for the compat ioctl interface uses
an intermediate ifc structure allocated in userland for the duration of
the syscall. Though, it fails to initialize the padding bytes inserted
for alignment and that for leaks four bytes of kernel stack. Add an
explicit memset(0) before filling the structure to avoid the info leak.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 2.6.32: adjust filename, context]
Signed-off-by: Willy Tarreau <w@1wt.eu>

fs/compat_ioctl.c

@@ -352,6 +352,7 @@ static int dev_ifconf(unsigned int fd, unsigned int cmd, unsigned long arg)
 	if (copy_from_user(&ifc32, compat_ptr(arg), sizeof(struct ifconf32)))
 		return -EFAULT;
 
+	memset(&ifc, 0, sizeof(ifc));
 	if (ifc32.ifcbuf == 0) {
 		ifc32.ifc_len = 0;
 		ifc.ifc_len = 0;