basic/audit-util: make a test request before enabling use of audit

If a container manager does not follow the guidance in
https://systemd.io/CONTAINER_INTERFACE/ regarding audit capabilities,
then the current check may not be sufficient to determine that audit
will function properly. In particular, when calling bind() on the audit
fd, we will get EPERM if running in a user-namespaced container.

Expand the check to make an AUDIT_GET_FEATURE request on the audit fd to
test if it is working. If this fails with ECONNREFUSED, we know it is
because the kernel does not support the use of audit outside of the
initial user namespace.

Note that the approach of this patch was suggested here:
https://github.com/systemd/systemd/pull/19443#issuecomment-829566659

Fixes: #6519

1 files changed, 60 insertions, 1 deletions

diff --git a/src/basic/audit-util.c b/src/basic/audit-util.c
index 1bf88b1e52..f2dce206b3 100644
--- a/src/basic/audit-util.c
+++ b/src/basic/audit-util.c
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <errno.h>
+#include <linux/audit.h>
 #include <linux/netlink.h>
 #include <stdio.h>
 #include <sys/socket.h>
@@ -12,6 +13,7 @@
 #include "macro.h"
 #include "parse-util.h"
 #include "process-util.h"
+#include "socket-util.h"
 #include "user-util.h"
 
 int audit_session_from_pid(pid_t pid, uint32_t *id) {
@@ -68,8 +70,50 @@ int audit_loginuid_from_pid(pid_t pid, uid_t *uid) {
         return 0;
 }
 
+static int try_audit_request(int fd) {
+        struct iovec iov;
+        struct msghdr mh;
+        ssize_t n;
+
+        assert(fd >= 0);
+
+        struct {
+                struct nlmsghdr hdr;
+                struct nlmsgerr err;
+        } _packed_ msg = {
+                .hdr.nlmsg_len = NLMSG_LENGTH(0),
+                .hdr.nlmsg_type = AUDIT_GET_FEATURE,
+                .hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK,
+        };
+        iov = (struct iovec) {
+                .iov_base = &msg,
+                .iov_len = msg.hdr.nlmsg_len,
+        };
+        mh = (struct msghdr) {
+                .msg_iov = &iov,
+                .msg_iovlen = 1,
+        };
+
+        if (sendmsg(fd, &mh, MSG_NOSIGNAL) < 0)
+                return -errno;
+
+        iov.iov_len = sizeof(msg);
+
+        n = recvmsg_safe(fd, &mh, 0);
+        if (n < 0)
+                return -errno;
+        if (n != NLMSG_LENGTH(sizeof(struct nlmsgerr)))
+                return -EIO;
+
+        if (msg.hdr.nlmsg_type != NLMSG_ERROR)
+                return -EINVAL;
+
+        return msg.err.error;
+}
+
 bool use_audit(void) {
         static int cached_use = -1;
+        int r;
 
         if (cached_use < 0) {
                 int fd;
@@ -80,7 +124,22 @@ bool use_audit(void) {
                         if (!cached_use)
                                 log_debug_errno(errno, "Won't talk to audit: %m");
                 } else {
-                        cached_use = true;
+                        /* If we try and use the audit fd but get -ECONNREFUSED, it is because
+                         * we are not in the initial user namespace, and the kernel does not
+                         * have support for audit outside of the initial user namespace
+                         * (see https://elixir.bootlin.com/linux/latest/C/ident/audit_netlink_ok).
+                         *
+                         * If we receive any other error, do not disable audit because we are not
+                         * sure that the error indicates that audit will not work in general. */
+                        r = try_audit_request(fd);
+                        if (r < 0) {
+                                cached_use = r != -ECONNREFUSED;
+                                log_debug_errno(r, cached_use ?
+                                                   "Failed to make request on audit fd, ignoring: %m" :
+                                                   "Won't talk to audit: %m");
+                        } else
+                                cached_use = true;
+
                         safe_close(fd);
                 }
         }