commit 128394ef upstream.

Both damn things interpret userland pointers embedded into the payload;
worse, they are actually traversing those.  Leaving aside the bad
API design, this is very much _not_ safe to call with KERNEL_DS.
Bail out early if that happens.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>