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SubjectRe: [PATCH 18/27] bpf: Restrict kernel image access functions when the kernel is locked down
On Thu, Oct 19, 2017 at 03:52:49PM +0100, David Howells wrote:
> From: Chun-Yi Lee <jlee@suse.com>
>
> There are some bpf functions can be used to read kernel memory:
> bpf_probe_read, bpf_probe_write_user and bpf_trace_printk. These allow
> private keys in kernel memory (e.g. the hibernation image signing key) to
> be read by an eBPF program. Prohibit those functions when the kernel is
> locked down.
>
> Signed-off-by: Chun-Yi Lee <jlee@suse.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: netdev@vger.kernel.org
> ---
>
> kernel/trace/bpf_trace.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index dc498b605d5d..35e85a3fdb37 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -65,6 +65,11 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
> {
> int ret;
>
> + if (kernel_is_locked_down("BPF")) {
> + memset(dst, 0, size);
> + return -EPERM;
> + }

That doesn't help the lockdown purpose.
If you don't trust the root the only way to prevent bpf read
memory is to disable the whole thing.
Have a single check in sys_bpf() to disallow everything if kernel_is_locked_down()
and don't add overhead to critical path like bpf_probe_read().

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