Sasha's report:
	> While fuzzing with trinity inside a KVM tools guest running the latest -next
	> kernel with the KASAN patchset, I've stumbled on the following spew:
	>
	> [ 4448.949424] ==================================================================
	> [ 4448.951737] AddressSanitizer: user-memory-access on address 0
	> [ 4448.952988] Read of size 2 by thread T19638:
	> [ 4448.954510] CPU: 28 PID: 19638 Comm: trinity-c76 Not tainted 3.16.0-rc4-next-20140711-sasha-00046-g07d3099-dirty #813
	> [ 4448.956823]  ffff88046d86ca40 0000000000000000 ffff880082f37e78 ffff880082f37a40
	> [ 4448.958233]  ffffffffb6e47068 ffff880082f37a68 ffff880082f37a58 ffffffffb242708d
	> [ 4448.959552]  0000000000000000 ffff880082f37a88 ffffffffb24255b1 0000000000000000
	> [ 4448.961266] Call Trace:
	> [ 4448.963158] dump_stack (lib/dump_stack.c:52)
	> [ 4448.964244] kasan_report_user_access (mm/kasan/report.c:184)
	> [ 4448.965507] __asan_load2 (mm/kasan/kasan.c:352)
	> [ 4448.966482] ? netlink_sendmsg (net/netlink/af_netlink.c:2339)
	> [ 4448.967541] netlink_sendmsg (net/netlink/af_netlink.c:2339)
	> [ 4448.968537] ? get_parent_ip (kernel/sched/core.c:2555)
	> [ 4448.970103] sock_sendmsg (net/socket.c:654)
	> [ 4448.971584] ? might_fault (mm/memory.c:3741)
	> [ 4448.972526] ? might_fault (./arch/x86/include/asm/current.h:14 mm/memory.c:3740)
	> [ 4448.973596] ? verify_iovec (net/core/iovec.c:64)
	> [ 4448.974522] ___sys_sendmsg (net/socket.c:2096)
	> [ 4448.975797] ? put_lock_stats.isra.13 (./arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
	> [ 4448.977030] ? lock_release_holdtime (kernel/locking/lockdep.c:273)
	> [ 4448.978197] ? lock_release_non_nested (kernel/locking/lockdep.c:3434 (discriminator 1))
	> [ 4448.979346] ? check_chain_key (kernel/locking/lockdep.c:2188)
	> [ 4448.980535] __sys_sendmmsg (net/socket.c:2181)
	> [ 4448.981592] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2600)
	> [ 4448.982773] ? trace_hardirqs_on (kernel/locking/lockdep.c:2607)
	> [ 4448.984458] ? syscall_trace_enter (arch/x86/kernel/ptrace.c:1500 (discriminator 2))
	> [ 4448.985621] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2600)
	> [ 4448.986754] SyS_sendmmsg (net/socket.c:2201)
	> [ 4448.987708] tracesys (arch/x86/kernel/entry_64.S:542)
	> [ 4448.988929] ==================================================================

This reports means that we've come to netlink_sendmsg() with msg->msg_name == NULL and msg->msg_namelen > 0.

After this report there was no usual "Unable to handle kernel NULL pointer dereference"
and this gave me a clue that address 0 is mapped and contains valid socket address structure in it.

This bug was introduced in f3d33426
(net: rework recvmsg handler msg_name and msg_namelen logic).
Commit message states that:
	"Set msg->msg_name = NULL if user specified a NULL in msg_name but had a
	 non-null msg_namelen in verify_iovec/verify_compat_iovec. This doesn't
	 affect sendto as it would bail out earlier while trying to copy-in the
	 address."
But in fact this affects sendto when address 0 is mapped and contains
socket address structure in it. In such case copy-in address will succeed,
verify_iovec() function will successfully exit with msg->msg_namelen > 0
and msg->msg_name == NULL.

This patch fixes it by setting msg_namelen to 0 if msg_name == NULL.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: <stable@vger.kernel.org>
Reported-by: NSasha Levin <sasha.levin@oracle.com>
Signed-off-by: NAndrey Ryabinin <a.ryabinin@samsung.com>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>