Subject: [PATCH bpf v3 2/2] selftests/bpf: make sure optvals > PAGE_SIZE are bypassed

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 8 Jun 2020 11:27:48 -0700

We are relying on the fact, that we can pass > sizeof(int) optvals
to the SOL_IP+IP_FREEBIND option (the kernel will take first 4 bytes).
In the BPF program, we return EPERM if optval is greater than optval_end
(implemented via PTR_TO_PACKET/PTR_TO_PACKET_END) and rely on the verifier
to enforce the fact that this data can not be touched.

Signed-off-by: Stanislav Fomichev <[email-masked]>
---
 .../selftests/bpf/prog_tests/sockopt_sk.c     | 26 +++++++++++++++++++
 .../testing/selftests/bpf/progs/sockopt_sk.c  | 20 ++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c b/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
index 2061a6beac0f..eae1c8a1fee0 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockopt_sk.c
@@ -13,6 +13,7 @@ static int getsetsockopt(void)
 		char cc[16]; /* TCP_CA_NAME_MAX */
 	} buf = {};
 	socklen_t optlen;
+	char *big_buf;
 
 	fd = socket(AF_INET, SOCK_STREAM, 0);
 	if (fd < 0) {
@@ -78,6 +79,31 @@ static int getsetsockopt(void)
 		goto err;
 	}
 
+	/* IP_FREEBIND - BPF can't access optval when optlen > PAGE_SIZE */
+
+	optlen = getpagesize() * 2;
+	big_buf = calloc(1, optlen);
+	if (!big_buf) {
+		log_err("Couldn't allocate two pages");
+		goto err;
+	}
+
+	err = setsockopt(fd, SOL_IP, IP_FREEBIND, big_buf, optlen);
+	if (err != 0) {
+		log_err("Failed to call setsockopt, ret=%d", err);
+		free(big_buf);
+		goto err;
+	}
+
+	err = getsockopt(fd, SOL_IP, IP_FREEBIND, big_buf, &optlen);
+	if (err != 0) {
+		log_err("Failed to call getsockopt, ret=%d", err);
+		free(big_buf);
+		goto err;
+	}
+
+	free(big_buf);
+
 	/* SO_SNDBUF is overwritten */
 
 	buf.u32 = 0x01010101;
diff --git a/tools/testing/selftests/bpf/progs/sockopt_sk.c b/tools/testing/selftests/bpf/progs/sockopt_sk.c
index d5a5eeb5fb52..933a2ef9c930 100644
--- a/tools/testing/selftests/bpf/progs/sockopt_sk.c
+++ b/tools/testing/selftests/bpf/progs/sockopt_sk.c
@@ -51,6 +51,16 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
+		if (optval > optval_end) {
+			/* For optval > PAGE_SIZE, the actual data
+			 * is not provided.
+			 */
+			return 0; /* EPERM, unexpected data size */
+		}
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 
@@ -112,6 +122,16 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
+		if (optval > optval_end) {
+			/* For optval > PAGE_SIZE, the actual data
+			 * is not provided.
+			 */
+			return 0; /* EPERM, unexpected data size */
+		}
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 
-- 
2.27.0.278.ge193c7cf3a9-goog