audit-util: return -ENODATA from audit_{session|loginuid}_from_pid() if invoked in a container (#35072)

The auditing subsystem is still not virtualized for containers, hence
the two values don't really make sense inside them, they will just leak
information from outside into the container. Hence don't make use of the
data if we detect we are run inside of a container.

This has visible effects: logind will no longer try to reuse the
auditing session ids as its own session ids when run inside a container.

While are at it, modernize the calls in more ways:

1. switch to pidref behaviour, all but one of our uses are using pidref
anyway already.
2. use read_virtual_file() + proc_mounted()
3. reasonably distinguish ENOENT errors when reading the process proc
files: distinguish the case where /proc is not mounted, from the case
where the process is already gone, from where auditing is not enabled in
the kernel build.

src/basic/audit-util.c

@@ -15,27 +15,59 @@
 #include "parse-util.h"
 #include "process-util.h"
 #include "socket-util.h"
+#include "stat-util.h"
 #include "user-util.h"
+#include "virt.h"
 
-int audit_session_from_pid(pid_t pid, uint32_t *id) {
-        _cleanup_free_ char *s = NULL;
-        const char *p;
-        uint32_t u;
+static int audit_read_field(const PidRef *pid, const char *field, char **ret) {
         int r;
 
-        assert(id);
+        assert(field);
+        assert(ret);
 
-        /* We don't convert ENOENT to ESRCH here, since we can't
-         * really distinguish between "audit is not available in the
-         * kernel" and "the process does not exist", both which will
-         * result in ENOENT. */
+        if (!pidref_is_set(pid))
+                return -ESRCH;
 
-        p = procfs_file_alloca(pid, "sessionid");
+        /* Auditing is currently not virtualized for containers. Let's hence not use the audit session ID or
+         * login UID for now, it will be leaked in from the host */
+        if (detect_container() > 0)
+                return -ENODATA;
 
-        r = read_one_line_file(p, &s);
+        const char *p = procfs_file_alloca(pid->pid, field);
+
+        _cleanup_free_ char *s = NULL;
+        bool enoent = false;
+        r = read_virtual_file(p, SIZE_MAX, &s, /* ret_size= */ NULL);
+        if (r == -ENOENT) {
+                if (proc_mounted() == 0)
+                        return -ENOSYS;
+                enoent = true;
+        } else if (r < 0)
+                return r;
+
+        r = pidref_verify(pid);
         if (r < 0)
                 return r;
 
+        if (enoent) /* We got ENOENT, but /proc/ was mounted and the PID still valid? In that case it appears
+                     * auditing is not supported by the kernel. */
+                return -ENODATA;
+
+        delete_trailing_chars(s, NEWLINE);
+
+        *ret = TAKE_PTR(s);
+        return 0;
+}
+
+int audit_session_from_pid(const PidRef *pid, uint32_t *ret_id) {
+        _cleanup_free_ char *s = NULL;
+        int r;
+
+        r = audit_read_field(pid, "sessionid", &s);
+        if (r < 0)
+                return r;
+
+        uint32_t u;
         r = safe_atou32(s, &u);
         if (r < 0)
                 return r;
@@ -43,32 +75,24 @@ int audit_session_from_pid(pid_t pid, uint32_t *id) {
         if (!audit_session_is_valid(u))
                 return -ENODATA;
 
-        *id = u;
+        if (ret_id)
+                *ret_id = u;
+
         return 0;
 }
 
-int audit_loginuid_from_pid(pid_t pid, uid_t *uid) {
+int audit_loginuid_from_pid(const PidRef *pid, uid_t *ret_uid) {
         _cleanup_free_ char *s = NULL;
-        const char *p;
-        uid_t u;
         int r;
 
-        assert(uid);
-
-        p = procfs_file_alloca(pid, "loginuid");
-
-        r = read_one_line_file(p, &s);
+        r = audit_read_field(pid, "loginuid", &s);
         if (r < 0)
                 return r;
 
-        r = parse_uid(s, &u);
-        if (r == -ENXIO) /* the UID was -1 */
+        if (streq(s, "4294967295")) /* loginuid as 4294967295 means not part of any session. */
                 return -ENODATA;
-        if (r < 0)
-                return r;
 
-        *uid = u;
-        return 0;
+        return parse_uid(s, ret_uid);
 }
 
 static int try_audit_request(int fd) {
@@ -113,33 +137,32 @@ bool use_audit(void) {
         static int cached_use = -1;
         int r;
 
-        if (cached_use < 0) {
-                int fd;
+        if (cached_use >= 0)
+                return cached_use;
 
-                fd = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC|SOCK_NONBLOCK, NETLINK_AUDIT);
-                if (fd < 0) {
-                        cached_use = !IN_SET(errno, EAFNOSUPPORT, EPROTONOSUPPORT, EPERM);
-                        if (!cached_use)
-                                log_debug_errno(errno, "Won't talk to audit: %m");
-                } else {
-                        /* 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);
-                }
+        _cleanup_close_ int fd = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC|SOCK_NONBLOCK, NETLINK_AUDIT);
+        if (fd < 0) {
+                cached_use = !ERRNO_IS_PRIVILEGE(errno) && !ERRNO_IS_NOT_SUPPORTED(errno);
+                if (cached_use)
+                        log_debug_errno(errno, "Unexpected error while creating audit socket, proceeding with its use: %m");
+                else
+                        log_debug_errno(errno, "Won't talk to audit, because feature or privilege absent: %m");
+        } else {
+                /* 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;
         }
 
         return cached_use;

src/basic/audit-util.h

@@ -5,10 +5,12 @@
 #include <stdint.h>
 #include <sys/types.h>
 
+#include "pidref.h"
+
 #define AUDIT_SESSION_INVALID UINT32_MAX
 
-int audit_session_from_pid(pid_t pid, uint32_t *id);
-int audit_loginuid_from_pid(pid_t pid, uid_t *uid);
+int audit_session_from_pid(const PidRef *pid, uint32_t *id);
+int audit_loginuid_from_pid(const PidRef *pid, uid_t *uid);
 
 bool use_audit(void);
 

src/basic/process-util.c

@@ -1815,6 +1815,9 @@ int namespace_fork(
 int set_oom_score_adjust(int value) {
         char t[DECIMAL_STR_MAX(int)];
 
+        if (!oom_score_adjust_is_valid(value))
+                return -EINVAL;
+
         xsprintf(t, "%i", value);
 
         return write_string_file("/proc/self/oom_score_adj", t,
@@ -1831,11 +1834,16 @@ int get_oom_score_adjust(int *ret) {
 
         delete_trailing_chars(t, WHITESPACE);
 
-        assert_se(safe_atoi(t, &a) >= 0);
-        assert_se(oom_score_adjust_is_valid(a));
+        r = safe_atoi(t, &a);
+        if (r < 0)
+                return r;
+
+        if (!oom_score_adjust_is_valid(a))
+                return -ENODATA;
 
         if (ret)
                 *ret = a;
+
         return 0;
 }
 

src/journal/journald-context.c

@@ -526,8 +526,8 @@ static void client_context_really_refresh(
         client_context_read_basic(c);
         (void) client_context_read_label(c, label, label_size);
 
-        (void) audit_session_from_pid(c->pid, &c->auditid);
-        (void) audit_loginuid_from_pid(c->pid, &c->loginuid);
+        (void) audit_session_from_pid(&PIDREF_MAKE_FROM_PID(c->pid), &c->auditid);
+        (void) audit_loginuid_from_pid(&PIDREF_MAKE_FROM_PID(c->pid), &c->loginuid);
 
         (void) client_context_read_cgroup(s, c, unit_id);
         (void) client_context_read_invocation_id(s, c);

src/libsystemd/sd-bus/bus-creds.c

@@ -1118,7 +1118,7 @@ int bus_creds_add_more(sd_bus_creds *c, uint64_t mask, PidRef *pidref, pid_t tid
         }
 
         if (missing & SD_BUS_CREDS_AUDIT_SESSION_ID) {
-                r = audit_session_from_pid(pidref->pid, &c->audit_session_id);
+                r = audit_session_from_pid(pidref, &c->audit_session_id);
                 if (r == -ENODATA) {
                         /* ENODATA means: no audit session id assigned */
                         c->audit_session_id = AUDIT_SESSION_INVALID;
@@ -1131,7 +1131,7 @@ int bus_creds_add_more(sd_bus_creds *c, uint64_t mask, PidRef *pidref, pid_t tid
         }
 
         if (missing & SD_BUS_CREDS_AUDIT_LOGIN_UID) {
-                r = audit_loginuid_from_pid(pidref->pid, &c->audit_login_uid);
+                r = audit_loginuid_from_pid(pidref, &c->audit_login_uid);
                 if (r == -ENODATA) {
                         /* ENODATA means: no audit login uid assigned */
                         c->audit_login_uid = UID_INVALID;

src/login/logind-dbus.c

@@ -1006,7 +1006,7 @@ static int create_session(
                                          "Maximum number of sessions (%" PRIu64 ") reached, refusing further sessions.",
                                          m->sessions_max);
 
-        (void) audit_session_from_pid(leader.pid, &audit_id);
+        (void) audit_session_from_pid(&leader, &audit_id);
         if (audit_session_is_valid(audit_id)) {
                 /* Keep our session IDs and the audit session IDs in sync */
 

src/login/logind-session.c

@@ -254,7 +254,7 @@ int session_set_leader_consume(Session *s, PidRef _leader) {
                         s->leader_fd_saved = true;
         }
 
-        (void) audit_session_from_pid(s->leader.pid, &s->audit_id);
+        (void) audit_session_from_pid(&s->leader, &s->audit_id);
 
         return 1;
 }

src/test/meson.build

@@ -46,6 +46,7 @@ simple_tests += files(
         'test-alloc-util.c',
         'test-architecture.c',
         'test-argv-util.c',
+        'test-audit-util.c',
         'test-barrier.c',
         'test-bitfield.c',
         'test-bitmap.c',

src/test/test-audit-util.c

@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "audit-util.h"
+#include "tests.h"
+
+TEST(audit_loginuid_from_pid) {
+        _cleanup_(pidref_done) PidRef self = PIDREF_NULL, pid1 = PIDREF_NULL;
+        int r;
+
+        assert_se(pidref_set_self(&self) >= 0);
+        assert_se(pidref_set_pid(&pid1, 1) >= 0);
+
+        uid_t uid;
+        r = audit_loginuid_from_pid(&self, &uid);
+        assert_se(r >= 0 || r == -ENODATA);
+        if (r >= 0)
+                log_info("self audit login uid: " UID_FMT, uid);
+
+        assert_se(audit_loginuid_from_pid(&pid1, &uid) == -ENODATA);
+
+        uint32_t sessionid;
+        r = audit_session_from_pid(&self, &sessionid);
+        assert_se(r >= 0 || r == -ENODATA);
+        if (r >= 0)
+                log_info("self audit session id: %" PRIu32, sessionid);
+
+        assert_se(audit_session_from_pid(&pid1, &sessionid) == -ENODATA);
+}
+
+static int intro(void) {
+        log_show_color(true);
+        return EXIT_SUCCESS;
+}
+
+DEFINE_TEST_MAIN_WITH_INTRO(LOG_INFO, intro);