privsep: Send all log messages to the privileged actioneer

If dhcpcd starts and no syslogd implementation is running then
various syscall filters could be triggered when dhcpcd wants to syslog
and it's already in a chroot.

Not all libc openlog implementations support LOG_NDELAY and
openlog does not return an error code and can also mask errno back to 0.
So we have no way of knowing if we have a syslog connection or not.
This means we cannot cache the connection at startup because syslog itself
will try and open if no connection.

As such, all logging is now directed to the dhcpcd privileged actioneer
process which will handle all the syslog and log file writing actions.

The only downside of this approach (other than an extra fd per process)
is that we no longer know which PID raised the message. While we could
put the correct PID in the logfile as we control the API, we cannot
put it into syslog as we cannot control that API.
As all privsep errors should log which function they came from this
will hopefully not be an issue as on the happy path only the master
process will log stuff.

6 files changed, 117 insertions, 24 deletions

diff --git a/src/dhcpcd.c b/src/dhcpcd.c
index 9b0a79a4..bd4cb74f 100644
--- a/src/dhcpcd.c
+++ b/src/dhcpcd.c
@@ -1860,7 +1860,7 @@ main(int argc, char **argv, char **envp)
 	ctx.dhcp6_wfd = -1;
 #endif
 #ifdef PRIVSEP
-	ctx.ps_root_fd = ctx.ps_data_fd = -1;
+	ctx.ps_root_fd = ctx.ps_syslog_fd = ctx.ps_data_fd = -1;
 	ctx.ps_inet_fd = ctx.ps_control_fd = -1;
 	TAILQ_INIT(&ctx.ps_processes);
 #endif
diff --git a/src/dhcpcd.h b/src/dhcpcd.h
index 8cdd3c19..b3cdf941 100644
--- a/src/dhcpcd.h
+++ b/src/dhcpcd.h
@@ -199,6 +199,7 @@ struct dhcpcd_ctx {
 	struct passwd *ps_user;	/* struct passwd for privsep user */
 	pid_t ps_root_pid;
 	int ps_root_fd;		/* Privileged Actioneer commands */
+	int ps_syslog_fd;	/* syslog(3) wrapper */
 	int ps_data_fd;		/* Data from root spawned processes */
 	struct eloop *ps_eloop;	/* eloop for polling root data */
 	struct ps_process_head ps_processes;	/* List of spawned processes */
diff --git a/src/logerr.c b/src/logerr.c
index ad337ba0..6d793082 100644
--- a/src/logerr.c
+++ b/src/logerr.c
@@ -47,11 +47,15 @@
 #undef LOGERR_TAG
 #endif
 
+/* syslog protocol is 1k message max, RFC 3164 section 4.1 */
+#define LOGERR_SYSLOGBUF	1024 + sizeof(int)
+
 #define UNUSED(a)		(void)(a)
 
 struct logctx {
 	char		 log_buf[BUFSIZ];
 	unsigned int	 log_opts;
+	int		 log_syslogfd;
 #ifndef SMALL
 	FILE		*log_file;
 #ifdef LOGERR_TAG
@@ -63,9 +67,10 @@ struct logctx {
 static struct logctx _logctx = {
 	/* syslog style, but without the hostname or tag. */
 	.log_opts = LOGERR_LOG | LOGERR_LOG_DATE | LOGERR_LOG_PID,
+	.log_syslogfd = -1,
 };
 
-#if defined(LOGERR_TAG) && defined(__linux__)
+#if defined(__linux__)
 /* Poor man's getprogname(3). */
 static char *_logprog;
 static const char *
@@ -79,9 +84,12 @@ getprogname(void)
 		 * so zero the buffer. */
 		if ((_logprog = calloc(1, PATH_MAX + 1)) == NULL)
 			return NULL;
+		if (readlink("/proc/self/exe", _logprog, PATH_MAX + 1) == -1) {
+			free(_logprog);
+			_logprog = NULL;
+			return NULL;
+		}
 	}
-	if (readlink("/proc/self/exe", _logprog, PATH_MAX + 1) == -1)
-		return NULL;
 	if (_logprog[0] == '[')
 		return NULL;
 	p = strrchr(_logprog, '/');
@@ -198,22 +206,33 @@ vlogmessage(int pri, const char *fmt, va_list args)
 	struct logctx *ctx = &_logctx;
 	int len = 0;
 
+	if (ctx->log_syslogfd != -1) {
+		char buf[LOGERR_SYSLOGBUF];
+
+		memcpy(buf, &pri, sizeof(pri));
+		len = vsnprintf(buf + sizeof(pri), sizeof(buf) - sizeof(pri),
+		    fmt, args);
+		if (len != -1)
+			len = (int)write(ctx->log_syslogfd, buf,
+			    ((size_t)++len) + sizeof(pri));
+		return len;
+	}
+
 	if (ctx->log_opts & LOGERR_ERR &&
 	    (pri <= LOG_ERR ||
 	    (!(ctx->log_opts & LOGERR_QUIET) && pri <= LOG_INFO) ||
 	    (ctx->log_opts & LOGERR_DEBUG && pri <= LOG_DEBUG)))
 		len = vlogprintf_r(ctx, stderr, fmt, args);
 
-	if (!(ctx->log_opts & LOGERR_LOG))
-		return len;
-
 #ifndef SMALL
 	if (ctx->log_file != NULL &&
 	    (pri != LOG_DEBUG || (ctx->log_opts & LOGERR_DEBUG)))
 		len = vlogprintf_r(ctx, ctx->log_file, fmt, args);
 #endif
 
-	vsyslog(pri, fmt, args);
+	if (ctx->log_opts & LOGERR_LOG)
+		vsyslog(pri, fmt, args);
+
 	return len;
 }
 #if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 5))
@@ -331,6 +350,43 @@ log_errx(const char *fmt, ...)
 	va_end(args);
 }
 
+int
+loggetsyslogfd(void)
+{
+	struct logctx *ctx = &_logctx;
+
+	return ctx->log_syslogfd;
+}
+
+void
+logsetsyslogfd(int fd)
+{
+	struct logctx *ctx = &_logctx;
+
+	ctx->log_syslogfd = fd;
+}
+
+int
+loghandlesyslogfd(int fd)
+{
+	char buf[LOGERR_SYSLOGBUF];
+	int len, pri;
+
+	len = (int)read(fd, buf, sizeof(buf));
+	if (len == -1)
+		return -1;
+
+	/* Ensure we have pri and a terminator */
+	if (len < (int)sizeof(pri) + 1 || buf[len - 1] != '\0') {
+		errno = EINVAL;
+		return -1;
+	}
+
+	memcpy(&pri, buf, sizeof(pri));
+	logmessage(pri, "%s", buf + sizeof(pri));
+	return len;
+}
+
 unsigned int
 loggetopts(void)
 {
@@ -373,15 +429,9 @@ logopen(const char *path)
 
 	(void)setvbuf(stderr, ctx->log_buf, _IOLBF, sizeof(ctx->log_buf));
 
-	if (!(ctx->log_opts & LOGERR_LOG))
-		return 1;
-
-#ifdef LOG_NDELAY
-	opts |= LOG_NDELAY;
-#endif
 	if (ctx->log_opts & LOGERR_LOG_PID)
 		opts |= LOG_PID;
-	openlog(NULL, opts, LOGERR_SYSLOG_FACILITY);
+	openlog(getprogname(), opts, LOGERR_SYSLOG_FACILITY);
 	if (path == NULL)
 		return 1;
 
@@ -410,7 +460,7 @@ logclose(void)
 	fclose(ctx->log_file);
 	ctx->log_file = NULL;
 #endif
-#if defined(LOGERR_TAG) && defined(__linux__)
+#if defined(__linux__)
 	free(_logprog);
 #endif
 }
diff --git a/src/logerr.h b/src/logerr.h
index 4b4d6dc4..c1b80153 100644
--- a/src/logerr.h
+++ b/src/logerr.h
@@ -76,6 +76,11 @@ __printflike(2, 3) void logerrmessage(int pri, const char *fmt, ...);
 #define logerr(...)	log_err(__VA_ARGS__)
 #define logerrx(...)	log_errx(__VA_ARGS__)
 
+/* For syslog in a chroot */
+int loggetsyslogfd(void);
+void logsetsyslogfd(int);
+int loghandlesyslogfd(int);
+
 unsigned int loggetopts(void);
 void logsetopts(unsigned int);
 #define	LOGERR_DEBUG	(1U << 6)
diff --git a/src/privsep-root.c b/src/privsep-root.c
index 770dd953..209f2624 100644
--- a/src/privsep-root.c
+++ b/src/privsep-root.c
@@ -780,18 +780,34 @@ ps_root_dispatch(void *arg)
 		logerr(__func__);
 }
 
+static void
+ps_root_syslog(void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	if (loghandlesyslogfd(ctx->ps_syslog_fd) == -1)
+		logerr(__func__);
+}
+
 pid_t
 ps_root_start(struct dhcpcd_ctx *ctx)
 {
-	int fd[2];
+	int logfd[2], datafd[2];
 	pid_t pid;
 
-	if (socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CXNB, 0, fd) == -1)
+	if (xsocketpair(AF_UNIX, SOCK_DGRAM | SOCK_CXNB, 0, logfd) == -1)
 		return -1;
-	if (ps_setbuf_fdpair(fd) == -1)
+#ifdef PRIVSEP_RIGHTS
+	if (ps_rights_limit_fdpair(logfd) == -1)
+		return -1;
+#endif
+
+	if (socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CXNB, 0, datafd) == -1)
+		return -1;
+	if (ps_setbuf_fdpair(datafd) == -1)
 		return -1;
 #ifdef PRIVSEP_RIGHTS
-	if (ps_rights_limit_fdpair(fd) == -1)
+	if (ps_rights_limit_fdpair(datafd) == -1)
 		return -1;
 #endif
 
@@ -800,14 +816,22 @@ ps_root_start(struct dhcpcd_ctx *ctx)
 	    ps_root_startcb, ps_root_signalcb, 0);
 
 	if (pid == 0) {
-		ctx->ps_data_fd = fd[1];
-		close(fd[0]);
+		ctx->ps_syslog_fd = logfd[1];
+		if (eloop_event_add(ctx->eloop, ctx->ps_syslog_fd,
+		    ps_root_syslog, ctx) == -1)
+			return -1;
+		close(logfd[0]);
+		ctx->ps_data_fd = datafd[1];
+		close(datafd[0]);
 		return 0;
 	} else if (pid == -1)
 		return -1;
 
-	ctx->ps_data_fd = fd[0];
-	close(fd[1]);
+	logsetsyslogfd(logfd[0]);
+	close(logfd[1]);
+
+	ctx->ps_data_fd = datafd[0];
+	close(datafd[1]);
 	if (eloop_event_add(ctx->eloop, ctx->ps_data_fd,
 	    ps_root_dispatch, ctx) == -1)
 		return -1;
diff --git a/src/privsep.c b/src/privsep.c
index e9fa463f..6e76902a 100644
--- a/src/privsep.c
+++ b/src/privsep.c
@@ -542,6 +542,19 @@ ps_mastersandbox(struct dhcpcd_ctx *ctx, const char *_pledge)
 	dropped = ps_dropprivs(ctx);
 	if (forked)
 		ctx->options |= DHCPCD_FORKED;
+
+	/*
+	 * If we don't have a root process, we cannot use syslog.
+	 * If it cannot be opened before chrooting then syslog(3) will fail.
+	 * openlog(3) does not return an error which doubly sucks.
+	 */
+	if (ctx->ps_root_fd == -1) {
+		unsigned int logopts = loggetopts();
+
+		logopts &= ~LOGERR_LOG;
+		logsetopts(logopts);
+	}
+
 	if (dropped == -1) {
 		logerr("%s: ps_dropprivs", __func__);
 		return -1;