Re: [PATCH V34 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

Kees Cook
2019-06-23T00:09:42Z

On Fri, Jun 21, 2019 at 05:03:52PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.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 and kernel memory to be altered without
> restriction. Disable them if the kernel has been locked down in
> confidentiality mode.
>
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: David Howells <dhowells@redhat.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: Matthew Garrett <mjg59@google.com>
> cc: netdev@vger.kernel.org
> cc: Chun-Yi Lee <jlee@suse.com>
> cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> ---
> include/linux/security.h | 1 +
> kernel/trace/bpf_trace.c | 20 +++++++++++++++++++-
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index e6e3e2403474..de0d37b1fe79 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -97,6 +97,7 @@ enum lockdown_reason {
> LOCKDOWN_INTEGRITY_MAX,
> LOCKDOWN_KCORE,
> LOCKDOWN_KPROBES,
> + LOCKDOWN_BPF_READ,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index d64c00afceb5..638f9b00a8df 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -137,6 +137,10 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
> {
> int ret;
>
> + ret = security_locked_down(LOCKDOWN_BPF_READ);
> + if (ret)
> + return ret;
> +
> ret = probe_kernel_read(dst, unsafe_ptr, size);
> if (unlikely(ret < 0))
> memset(dst, 0, size);
> @@ -156,6 +160,12 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
> BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const void *, src,
> u32, size)
> {
> + int ret;
> +
> + ret = security_locked_down(LOCKDOWN_BPF_READ);
> + if (ret)
> + return ret;
> +
> /*
> * Ensure we're in user context which is safe for the helper to
> * run. This helper has no business in a kthread.
> @@ -205,7 +215,11 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
> int fmt_cnt = 0;
> u64 unsafe_addr;
> char buf[64];
> - int i;
> + int i, ret;
> +
> + ret = security_locked_down(LOCKDOWN_BPF_READ);
> + if (ret)
> + return ret;
>
> /*
> * bpf_check()->check_func_arg()->check_stack_boundary()
> @@ -534,6 +548,10 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
> {
> int ret;
>
> + ret = security_locked_down(LOCKDOWN_BPF_READ);
> + if (ret)
> + return ret;
> +
> /*
> * The strncpy_from_unsafe() call will likely not fill the entire
> * buffer, but that's okay in this circumstance as we're probing
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 5a08c17f224d..2eea2cc13117 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -33,6 +33,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_KCORE] = "/proc/kcore access",
> [LOCKDOWN_KPROBES] = "use of kprobes",
> + [LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
>
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook