lsm: fix default return value for inode_getsecctx

stable inclusion
from stable-v6.6.3
commit 37dab33f754abd24b384d2b7b07349dc6611381b
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LBQP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37dab33f754abd24b384d2b7b07349dc6611381b



--------------------------------

commit b36995b8609a5a8fe5cf259a1ee768fcaed919f8 upstream.

-EOPNOTSUPP is the return value that implements a "no-op" hook, not 0.

Without this fix having only the BPF LSM enabled (with no programs
attached) can cause uninitialized variable reads in
nfsd4_encode_fattr(), because the BPF hook returns 0 without touching
the 'ctxlen' variable and the corresponding 'contextlen' variable in
nfsd4_encode_fattr() remains uninitialized, yet being treated as valid
based on the 0 return value.

Cc: stable@vger.kernel.org
Fixes: 98e828a0 ("security: Refactor declaration of LSM hooks")
Reported-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>