[PATCH v2 4/4] samples: Add example of using PTRACE_GETFD in conjunction with user trap

[Sargun Dhillon <sargun@sargun.me>]

This sample adds the usage of SECCOMP_RET_USER_NOTIF together with ptrace
PTRACE_GETFD. It shows trapping a syscall, and handling it by extracting
the FD into the parent process without stopping the child process.
Although, in this example, there's no explicit policy separation in
the two processes, it can be generalized into the example of a transparent
proxy.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 samples/seccomp/.gitignore         |   1 +
 samples/seccomp/Makefile           |   9 +-
 samples/seccomp/user-trap-ptrace.c | 193 +++++++++++++++++++++++++++++
 3 files changed, 202 insertions(+), 1 deletion(-)
 create mode 100644 samples/seccomp/user-trap-ptrace.c

diff --git a/samples/seccomp/.gitignore b/samples/seccomp/.gitignore
index d1e2e817d556..169bc130ec39 100644
--- a/samples/seccomp/.gitignore
+++ b/samples/seccomp/.gitignore
@@ -2,3 +2,4 @@ bpf-direct
 bpf-fancy
 dropper
 user-trap
+user-trap-ptrace
diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
index 82b7347318d1..c0f3ef713f5b 100644
--- a/samples/seccomp/Makefile
+++ b/samples/seccomp/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 ifndef CROSS_COMPILE
-hostprogs-y := bpf-fancy dropper bpf-direct user-trap
+hostprogs-y := bpf-fancy dropper bpf-direct user-trap user-trap-ptrace
 
 HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
@@ -24,6 +24,11 @@ HOSTCFLAGS_user-trap.o += -I$(objtree)/usr/include
 HOSTCFLAGS_user-trap.o += -idirafter $(objtree)/include
 user-trap-objs := user-trap.o user-trap-helper.o
 
+HOSTCFLAGS_user-trap-ptrace.o += -I$(objtree)/usr/include
+HOSTCFLAGS_user-trap-ptrace.o += -idirafter $(objtree)/include
+user-trap-ptrace-objs := user-trap-ptrace.o user-trap-helper.o
+
+
 # Try to match the kernel target.
 ifndef CONFIG_64BIT
 
@@ -39,10 +44,12 @@ HOSTCFLAGS_dropper.o += $(MFLAG)
 HOSTCFLAGS_bpf-helper.o += $(MFLAG)
 HOSTCFLAGS_bpf-fancy.o += $(MFLAG)
 HOSTCFLAGS_user-trap.o += $(MFLAG)
+HOSTCFLAGS_user-trap-ptrace.o += $(MFLAG)
 HOSTLDLIBS_bpf-direct += $(MFLAG)
 HOSTLDLIBS_bpf-fancy += $(MFLAG)
 HOSTLDLIBS_dropper += $(MFLAG)
 HOSTLDLIBS_user-trap += $(MFLAG)
+HOSTLDLIBS_user-trap-ptrace += $(MFLAG)
 endif
 always := $(hostprogs-y)
 endif
diff --git a/samples/seccomp/user-trap-ptrace.c b/samples/seccomp/user-trap-ptrace.c
new file mode 100644
index 000000000000..5cca1cb4916c
--- /dev/null
+++ b/samples/seccomp/user-trap-ptrace.c
@@ -0,0 +1,193 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/seccomp.h>
+#include <linux/ptrace.h>
+#include <linux/prctl.h>
+#include <sys/socket.h>
+#include <sys/prctl.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/ioctl.h>
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <netinet/in.h>
+#include "user-trap-helper.h"
+
+#define CHILD_PORT_TRY_BIND	80
+#define CHILD_PORT_ACTUAL_BIND	4998
+
+static int ptrace(long request, long pid, void *addr, unsigned long data)
+{
+	errno = 0;
+	return syscall(__NR_ptrace, request, pid, addr, data);
+}
+
+static int ptrace_getfd(long pid, struct ptrace_getfd_args *args)
+{
+	errno = 0;
+	return syscall(__NR_ptrace, PTRACE_GETFD, pid, sizeof(*args), args);
+}
+
+static int tracee(void)
+{
+	struct sockaddr_in addr = {
+		.sin_family	= AF_INET,
+		.sin_port	= htons(CHILD_PORT_TRY_BIND),
+		.sin_addr	= {
+			.s_addr	= htonl(INADDR_ANY)
+		}
+	};
+	socklen_t addrlen = sizeof(addr);
+	int sock, ret = 1;
+
+	sock = socket(AF_INET, SOCK_STREAM, 0);
+	if (sock == -1) {
+		perror("socket");
+		goto out;
+	}
+
+
+	if (bind(sock, (struct sockaddr *) &addr, sizeof(addr))) {
+		perror("bind");
+		goto out;
+	}
+
+	printf("Child successfully performed bind operation\n");
+	if (getsockname(sock, (struct sockaddr *) &addr, &addrlen)) {
+		perror("getsockname");
+		goto out;
+	}
+
+
+	printf("Socket bound to port %d\n", ntohs(addr.sin_port));
+	assert(ntohs(addr.sin_port) == CHILD_PORT_ACTUAL_BIND);
+
+	ret = 0;
+out:
+	return ret;
+}
+
+static int handle_req(int listener)
+{
+	struct sockaddr_in addr = {
+		.sin_family	= AF_INET,
+		.sin_port	= htons(4998),
+		.sin_addr	= {
+			.s_addr	= htonl(INADDR_LOOPBACK)
+		}
+	};
+	struct ptrace_getfd_args getfd_args = {
+		.options = PTRACE_GETFD_O_CLOEXEC
+	};
+	struct seccomp_notif_sizes sizes;
+	struct seccomp_notif_resp *resp;
+	struct seccomp_notif *req;
+	int fd, ret = 1;
+
+	if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
+		perror("seccomp(GET_NOTIF_SIZES)");
+		goto out;
+	}
+	req = malloc(sizes.seccomp_notif);
+	if (!req)
+		goto out;
+	memset(req, 0, sizeof(*req));
+
+	resp = malloc(sizes.seccomp_notif_resp);
+	if (!resp)
+		goto out_free_req;
+	memset(resp, 0, sizeof(*resp));
+
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
+		perror("ioctl recv");
+		goto out;
+	}
+	printf("Child tried to call bind with fd: %lld\n", req->data.args[0]);
+	getfd_args.fd = req->data.args[0];
+	fd = ptrace_getfd(req->pid, &getfd_args);
+	if (fd == -1) {
+		perror("ptrace_getfd");
+		goto out_free_resp;
+	}
+	if (bind(fd, (struct sockaddr *) &addr, sizeof(addr))) {
+		perror("bind");
+		goto out_free_resp;
+	}
+
+	resp->id = req->id;
+	resp->error = 0;
+	resp->val = 0;
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0) {
+		perror("ioctl send");
+		goto out_free_resp;
+	}
+
+	ret = 0;
+out_free_resp:
+	free(resp);
+out_free_req:
+	free(req);
+out:
+	return ret;
+}
+
+int main(void)
+{
+	int listener, sk_pair[2], ret = 1;
+	pid_t pid;
+
+	if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
+		perror("socketpair");
+		goto out;
+	}
+
+	pid = fork();
+	if (pid < 0) {
+		perror("fork");
+		goto close_pair;
+	}
+	if (pid == 0) {
+		if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+			perror("prctl(NO_NEW_PRIVS)");
+			exit(1);
+		}
+		listener = user_trap_syscall(__NR_bind,
+					     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+		if (listener < 0) {
+			perror("seccomp");
+			exit(1);
+		}
+		if (send_fd(sk_pair[1], listener) < 0)
+			exit(1);
+		close(listener);
+		exit(tracee());
+	}
+
+	if (ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_EXITKILL)) {
+		perror("ptrace(PTRACE_SEIZE)");
+		goto kill_child;
+	}
+
+	listener = recv_fd(sk_pair[0]);
+	if (listener < 0)
+		goto kill_child;
+
+	if (handle_req(listener))
+		goto kill_child;
+
+	/* Wait for child to finish */
+	waitpid(pid, NULL, 0);
+
+	ret = 0;
+	goto close_pair;
+
+kill_child:
+	kill(pid, SIGKILL);
+close_pair:
+	close(sk_pair[0]);
+	close(sk_pair[1]);
+out:
+	return ret;
+}
-- 
2.20.1