security: Fix the default value of secid_to_secctx hook

security_secid_to_secctx is called by the bpf_lsm hook and a successful
return value (i.e 0) implies that the parameter will be consumed by the
LSM framework. The current behaviour return success when the pointer
isn't initialized when CONFIG_BPF_LSM is enabled, with the default
return from kernel/bpf/bpf_lsm.c.

This is the internal error:

[ 1229.341488][ T2659] usercopy: Kernel memory exposure attempt detected from null address (offset 0, size 280)!
[ 1229.374977][ T2659] ------------[ cut here ]------------
[ 1229.376813][ T2659] kernel BUG at mm/usercopy.c:99!
[ 1229.378398][ T2659] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[ 1229.380348][ T2659] Modules linked in:
[ 1229.381654][ T2659] CPU: 0 PID: 2659 Comm: systemd-journal Tainted: G    B   W         5.7.0-rc5-next-20200511-00019-g864e0c6319b8-dirty #13
[ 1229.385429][ T2659] Hardware name: linux,dummy-virt (DT)
[ 1229.387143][ T2659] pstate: 80400005 (Nzcv daif +PAN -UAO BTYPE=--)
[ 1229.389165][ T2659] pc : usercopy_abort+0xc8/0xcc
[ 1229.390705][ T2659] lr : usercopy_abort+0xc8/0xcc
[ 1229.392225][ T2659] sp : ffff000064247450
[ 1229.393533][ T2659] x29: ffff000064247460 x28: 0000000000000000
[ 1229.395449][ T2659] x27: 0000000000000118 x26: 0000000000000000
[ 1229.397384][ T2659] x25: ffffa000127049e0 x24: ffffa000127049e0
[ 1229.399306][ T2659] x23: ffffa000127048e0 x22: ffffa000127048a0
[ 1229.401241][ T2659] x21: ffffa00012704b80 x20: ffffa000127049e0
[ 1229.403163][ T2659] x19: ffffa00012704820 x18: 0000000000000000
[ 1229.405094][ T2659] x17: 0000000000000000 x16: 0000000000000000
[ 1229.407008][ T2659] x15: 0000000000000000 x14: 003d090000000000
[ 1229.408942][ T2659] x13: ffff80000d5b25b2 x12: 1fffe0000d5b25b1
[ 1229.410859][ T2659] x11: 1fffe0000d5b25b1 x10: ffff80000d5b25b1
[ 1229.412791][ T2659] x9 : ffffa0001034bee0 x8 : ffff00006ad92d8f
[ 1229.414707][ T2659] x7 : 0000000000000000 x6 : ffffa00015eacb20
[ 1229.416642][ T2659] x5 : ffff0000693c8040 x4 : 0000000000000000
[ 1229.418558][ T2659] x3 : ffffa0001034befc x2 : d57a7483a01c6300
[ 1229.420610][ T2659] x1 : 0000000000000000 x0 : 0000000000000059
[ 1229.422526][ T2659] Call trace:
[ 1229.423631][ T2659]  usercopy_abort+0xc8/0xcc
[ 1229.425091][ T2659]  __check_object_size+0xdc/0x7d4
[ 1229.426729][ T2659]  put_cmsg+0xa30/0xa90
[ 1229.428132][ T2659]  unix_dgram_recvmsg+0x80c/0x930
[ 1229.429731][ T2659]  sock_recvmsg+0x9c/0xc0
[ 1229.431123][ T2659]  ____sys_recvmsg+0x1cc/0x5f8
[ 1229.432663][ T2659]  ___sys_recvmsg+0x100/0x160
[ 1229.434151][ T2659]  __sys_recvmsg+0x110/0x1a8
[ 1229.435623][ T2659]  __arm64_sys_recvmsg+0x58/0x70
[ 1229.437218][ T2659]  el0_svc_common.constprop.1+0x29c/0x340
[ 1229.438994][ T2659]  do_el0_svc+0xe8/0x108
[ 1229.440587][ T2659]  el0_svc+0x74/0x88
[ 1229.441917][ T2659]  el0_sync_handler+0xe4/0x8b4
[ 1229.443464][ T2659]  el0_sync+0x17c/0x180
[ 1229.444920][ T2659] Code: aa1703e2 aa1603e1 910a8260 97ecc860 (d4210000)
[ 1229.447070][ T2659] ---[ end trace 400497d91baeaf51 ]---
[ 1229.448791][ T2659] Kernel panic - not syncing: Fatal exception
[ 1229.450692][ T2659] Kernel Offset: disabled
[ 1229.452061][ T2659] CPU features: 0x240002,20002004
[ 1229.453647][ T2659] Memory Limit: none
[ 1229.455015][ T2659] ---[ end Kernel panic - not syncing: Fatal exception ]---

Rework the so the default return value is -EOPNOTSUPP.

There are likely other callbacks such as security_inode_getsecctx() that
may have the same problem, and that someone that understand the code
better needs to audit them.

Thank you Arnd for helping me figure out what went wrong.

Fixes: 98e828a0 ("security: Refactor declaration of LSM hooks")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/bpf/20200512174607.9630-1-anders.roxell@linaro.org