bpf: Disallow unprivileged bpf by default

Disabling unprivileged BPF would help prevent unprivileged users from
creating certain conditions required for potential speculative execution
side-channel attacks on unmitigated affected hardware.

A deep dive on such attacks and current mitigations is available here [0].

Sync with what many distros are currently applying already, and disable
unprivileged BPF by default. An admin can enable this at runtime, if
necessary, as described in 08389d8 ("bpf: Add kconfig knob for
disabling unpriv bpf by default").

  [0] "BPF and Spectre: Mitigating transient execution attacks", Daniel Borkmann, eBPF Summit '21
      https://ebpf.io/summit-2021-slides/eBPF_Summit_2021-Keynote-Daniel_Borkmann-BPF_and_Spectre.pdf

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/bpf/0ace9ce3f97656d5f62d11093ad7ee81190c3c25.1635535215.git.pawan.kumar.gupta@linux.intel.com

kernel/bpf/Kconfig

@@ -64,6 +64,7 @@ config BPF_JIT_DEFAULT_ON
 
 config BPF_UNPRIV_DEFAULT_OFF
 	bool "Disable unprivileged BPF by default"
+	default y
 	depends on BPF_SYSCALL
 	help
 	  Disables unprivileged BPF by default by setting the corresponding
@@ -72,6 +73,12 @@ config BPF_UNPRIV_DEFAULT_OFF
 	  disable it by setting it to 1 (from which no other transition to
 	  0 is possible anymore).
 
+	  Unprivileged BPF could be used to exploit certain potential
+	  speculative execution side-channel vulnerabilities on unmitigated
+	  affected hardware.
+
+	  If you are unsure how to answer this question, answer Y.
+
 source "kernel/bpf/preload/Kconfig"
 
 config BPF_LSM