sendmmsg/sendmsg: fix unsafe user pointer access

sendmmsg/sendmsg: fix unsafe user pointer access

Dereferencing a user pointer directly from kernel-space without going
through the copy_from_user family of functions is a bad idea. Two of
such usages can be found in the sendmsg code path called from sendmmsg,
added by

commit c71d8ebe7a4496fb7231151cb70a6baa0cb56f9a upstream.
commit 5b47b8038f183b44d2d8ff1c7d11a5c1be706b34 in the 3.0-stable tree.

Usages are performed through memcmp() and memcpy() directly. Fix those
by using the already copied msg_sys structure instead of the __user *msg
structure. Note that msg_sys can be set to NULL by verify_compat_iovec()
or verify_iovec(), which requires additional NULL pointer checks.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: David Goulet <[email protected]>
CC: Tetsuo Handa <[email protected]>
CC: Anton Blanchard <[email protected]>
CC: David S. Miller <[email protected]>
CC: stable <[email protected]>
Signed-off-by: David S. Miller <[email protected]>