RDS currently doesn't check if the length of the control message is
large enough to hold the required data, before dereferencing the control
message data. This results in following crash:

BUG: KASAN: stack-out-of-bounds in rds_rdma_bytes net/rds/send.c:1013
[inline]
BUG: KASAN: stack-out-of-bounds in rds_sendmsg+0x1f02/0x1f90
net/rds/send.c:1066
Read of size 8 at addr ffff8801c928fb70 by task syzkaller455006/3157

CPU: 0 PID: 3157 Comm: syzkaller455006 Not tainted 4.15.0-rc3+ #161
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
 rds_rdma_bytes net/rds/send.c:1013 [inline]
 rds_sendmsg+0x1f02/0x1f90 net/rds/send.c:1066
 sock_sendmsg_nosec net/socket.c:628 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:638
 ___sys_sendmsg+0x320/0x8b0 net/socket.c:2018
 __sys_sendmmsg+0x1ee/0x620 net/socket.c:2108
 SYSC_sendmmsg net/socket.c:2139 [inline]
 SyS_sendmmsg+0x35/0x60 net/socket.c:2134
 entry_SYSCALL_64_fastpath+0x1f/0x96
RIP: 0033:0x43fe49
RSP: 002b:00007fffbe244ad8 EFLAGS: 00000217 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fe49
RDX: 0000000000000001 RSI: 000000002020c000 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000217 R12: 00000000004017b0
R13: 0000000000401840 R14: 0000000000000000 R15: 0000000000000000

To fix this, we verify that the cmsg_len is large enough to hold the
data to be read, before proceeding further.
Reported-by: syzbot <syzkaller-bugs@googlegroups.com>
Signed-off-by: Avinash Repaka <avinash.repaka@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>