[PATCH V38 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

[Matthew Garrett <matthewgarrett@google.com>]

From: David Howells <dhowells@redhat.com>

bpf_read() and bpf_read_str() could potentially be abused to (eg) allow
private keys in kernel memory to be leaked. Disable them if the kernel
has been locked down in confidentiality mode.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
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     | 10 ++++++++++
 security/lockdown/lockdown.c |  1 +
 3 files changed, 12 insertions(+)

diff --git a/include/linux/security.h b/include/linux/security.h
index 987d8427f091..8dd1741a52cd 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -118,6 +118,7 @@ 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 ca1255d14576..492a8bfaae98 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -142,8 +142,13 @@ 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 < 0)
+		goto out;
+
 	ret = probe_kernel_read(dst, unsafe_ptr, size);
 	if (unlikely(ret < 0))
+out:
 		memset(dst, 0, size);
 
 	return ret;
@@ -569,6 +574,10 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
 {
 	int ret;
 
+	ret = security_locked_down(LOCKDOWN_BPF_READ);
+	if (ret < 0)
+		goto out;
+
 	/*
 	 * The strncpy_from_unsafe() call will likely not fill the entire
 	 * buffer, but that's okay in this circumstance as we're probing
@@ -580,6 +589,7 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
 	 */
 	ret = strncpy_from_unsafe(dst, unsafe_ptr, size);
 	if (unlikely(ret < 0))
+out:
 		memset(dst, 0, size);
 
 	return ret;
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 6b123cbf3748..1b89d3e8e54d 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",
 };
 
-- 
2.22.0.770.g0f2c4a37fd-goog