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

[Matthew Garrett]

From: David Howells <dhowells@xxxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx>
cc: netdev@xxxxxxxxxxxxxxx
cc: Chun-Yi Lee <jlee@xxxxxxxx>
cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
 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