[PATCH ebpf v2 2/2] bpf: Make unprivileged bpf depend on CONFIG_CPU_SPECTRE

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

Disabling unprivileged BPF would help prevent unprivileged users from
creating the conditions required for potential speculative execution
side-channel attacks on affected hardware. A deep dive on such attacks
and mitigation is available here [1].

If an architecture selects CONFIG_CPU_SPECTRE, disable unprivileged BPF
by default. An admin can enable this at runtime, if necessary.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

[1] https://ebpf.io/summit-2021-slides/eBPF_Summit_2021-Keynote-Daniel_Borkmann-BPF_and_Spectre.pdf
---
 kernel/bpf/Kconfig | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index a82d6de86522..510a5a73f9a2 100644
--- a/kernel/bpf/Kconfig
+++ b/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 if CPU_SPECTRE
 	depends on BPF_SYSCALL
 	help
 	  Disables unprivileged BPF by default by setting the corresponding
@@ -72,6 +73,10 @@ config BPF_UNPRIV_DEFAULT_OFF
 	  disable it by setting it to 1 (from which no other transition to
 	  0 is possible anymore).
 
+	  Unprivileged BPF can be used to exploit potential speculative
+	  execution side-channel vulnerabilities on affected hardware. If you
+	  are concerned about it, answer Y.
+
 source "kernel/bpf/preload/Kconfig"
 
 config BPF_LSM
-- 
2.31.1