On Thu, Oct 28, 2021 at 02:57:51PM +0100, Mark Rutland wrote:

On Wed, Oct 27, 2021 at 06:35:44PM -0700, Pawan Gupta wrote:

quoted

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

Why can't this just be "default y"?

Because not all arches are broken.

This series makes that the case on x86, and if SW is going to have to
deal with that we may as well do that everywhere, and say that on all
architectures we leave it to the sysadmin or kernel builder to optin to
permitting unprivileged BPF.

If we can change the default for x86 I see no reason we can't change
this globally, and we avoid tying this to CPU_SPECTRE specifically.

No, this is a spectre-like issue only, if you have hardware that does
not have these types of issues, why wouldn't this be ok to be disabled?

thanks,

greg k-h