bpf: Add lockdown check for probe_write_user helper

bpf: Add lockdown check for probe_write_user helper

Back then, commit 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper
to be called in tracers") added the bpf_probe_write_user() helper in order
to allow to override user space memory. Its original goal was to have a
facility to "debug, divert, and manipulate execution of semi-cooperative
processes" under CAP_SYS_ADMIN. Write to kernel was explicitly disallowed
since it would otherwise tamper with its integrity.

One use case was shown in cf9b1199de27 ("samples/bpf: Add test/example of
using bpf_probe_write_user bpf helper") where the program DNATs traffic
at the time of connect(2) syscall, meaning, it rewrites the arguments to
a syscall while they're still in userspace, and before the syscall has a
chance to copy the argument into kernel space. These days we have better
mechanisms in BPF for achieving the same (e.g. for load-balancers), but
without having to write to userspace memory.

Of course the bpf_probe_write_user() helper can also be used to abuse
many other things for both good or bad purpose. Outside of BPF, there is
a similar mechanism for ptrace(2) such as PTRACE_PEEK{TEXT,DATA} and
PTRACE_POKE{TEXT,DATA}, but would likely require some more effort.
Commit 96ae52279594 explicitly dedicated the helper for experimentation
purpose only. Thus, move the helper's availability behind a newly added
LOCKDOWN_BPF_WRITE_USER lockdown knob so that the helper is disabled under
the "integrity" mode. More fine-grained control can be implemented also
from LSM side with this change.

Fixes: 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper to be called in tracers")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>