bpf: Add kconfig knob for disabling unpriv bpf by default

commit 08389d8882 upstream.

Add a kconfig knob which allows for unprivileged bpf to be disabled by default.
If set, the knob sets /proc/sys/kernel/unprivileged_bpf_disabled to value of 2.

This still allows a transition of 2 -> {0,1} through an admin. Similarly,
this also still keeps 1 -> {1} behavior intact, so that once set to permanently
disabled, it cannot be undone aside from a reboot.

We've also added extra2 with max of 2 for the procfs handler, so that an admin
still has a chance to toggle between 0 <-> 2.

Either way, as an additional alternative, applications can make use of CAP_BPF
that we added a while ago.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/74ec548079189e4e4dffaeb42b8987bb3c852eee.1620765074.git.daniel@iogearbox.net
[fllinden@amazon.com: backported to 4.9]
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/sysctl/kernel.txt

@@ -90,6 +90,7 @@ show up in /proc/sys/kernel:
 - sysctl_writes_strict
 - tainted
 - threads-max
+- unprivileged_bpf_disabled
 - unknown_nmi_panic
 - watchdog
 - watchdog_thresh
@@ -995,6 +996,26 @@ available RAM pages threads-max is reduced accordingly.
 
 ==============================================================
 
+unprivileged_bpf_disabled:
+
+Writing 1 to this entry will disable unprivileged calls to bpf();
+once disabled, calling bpf() without CAP_SYS_ADMIN will return
+-EPERM. Once set to 1, this can't be cleared from the running kernel
+anymore.
+
+Writing 2 to this entry will also disable unprivileged calls to bpf(),
+however, an admin can still change this setting later on, if needed, by
+writing 0 or 1 to this entry.
+
+If BPF_UNPRIV_DEFAULT_OFF is enabled in the kernel config, then this
+entry will default to 2 instead of 0.
+
+  0 - Unprivileged calls to bpf() are enabled
+  1 - Unprivileged calls to bpf() are disabled without recovery
+  2 - Unprivileged calls to bpf() are disabled
+
+==============================================================
+
 unknown_nmi_panic:
 
 The value in this file affects behavior of handling NMI. When the