[COMPAT] net: SIOCGIFCONF data corruption

[COMPAT] net: SIOCGIFCONF data corruption

From: Alexandra Kossovsky <Alexandra.Kossovsky@oktetlabs.ru>

From http://bugzilla.kernel.org/show_bug.cgi?id=4746

There is user data corruption when using ioctl(SIOCGIFCONF) in 32-bit
application running amd64 kernel. I do not think that this problem is
exploitable, but any data corruption may lead to security problems.
Following code demonstrates the problem

#include <stdint.h>
#include <stdio.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <net/if.h>
#include <sys/ioctl.h>

char buf[256];

main()
{
	int s = socket(AF_INET, SOCK_DGRAM, 0);
	struct ifconf req;
	int i;

	req.ifc_buf = buf;
	req.ifc_len = 41;

	printf("Result %d\n", ioctl(s, SIOCGIFCONF, &req));
	printf("Len %d\n", req.ifc_len);
	for (i = 41; i < 256; i++)
		if (buf[i] != 0)
			printf("Byte %d is corrupted\n", i);
}

Steps to reproduce:
Compile the code above into 32-bit elf and run it. You'll get
Result 0
Len 32
Byte 48 is corrupted
Byte 52 is corrupted
Byte 53 is corrupted
Byte 54 is corrupted
Byte 55 is corrupted

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>