Re: [PATCH V34 22/29] Lock down tracing and perf kprobes when in confidentiality mode

Kees Cook
2019-06-23T00:09:05Z

On Fri, Jun 21, 2019 at 05:03:51PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.com>
>
> Disallow the creation of perf and ftrace kprobes when the kernel is
> locked down in confidentiality mode by preventing their registration.
> This prevents kprobes from being used to access kernel memory to steal
> crypto data, but continues to allow the use of kprobes from signed
> modules.
>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
> Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
> Cc: davem@davemloft.net
> Cc: Masami Hiramatsu <mhiramat@kernel.org>

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

-Kees

> ---
> include/linux/security.h | 1 +
> kernel/trace/trace_kprobe.c | 5 +++++
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 7 insertions(+)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 3875f6df2ecc..e6e3e2403474 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -96,6 +96,7 @@ enum lockdown_reason {
> LOCKDOWN_MMIOTRACE,
> LOCKDOWN_INTEGRITY_MAX,
> LOCKDOWN_KCORE,
> + LOCKDOWN_KPROBES,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
>
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 5d5129b05df7..5a76a0f79d48 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -11,6 +11,7 @@
> #include <linux/uaccess.h>
> #include <linux/rculist.h>
> #include <linux/error-injection.h>
> +#include <linux/security.h>
>
> #include "trace_dynevent.h"
> #include "trace_kprobe_selftest.h"
> @@ -415,6 +416,10 @@ static int __register_trace_kprobe(struct trace_kprobe *tk)
> {
> int i, ret;
>
> + ret = security_locked_down(LOCKDOWN_KPROBES);
> + if (ret)
> + return ret;
> +
> if (trace_probe_is_registered(&tk->tp))
> return -EINVAL;
>
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 4c9b324dfc55..5a08c17f224d 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -32,6 +32,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_MMIOTRACE] = "unsafe mmio",
> [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_KCORE] = "/proc/kcore access",
> + [LOCKDOWN_KPROBES] = "use of kprobes",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
>
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook