dynamic-user: don't use a UID that currently owns IPC objects (#6962)

This fixes a mostly theoretical potential security hole: if for some
reason we failed to remove IPC objects created for a dynamic user (maybe
because a MAC/SElinux erronously prohibited), then we should not hand
out the same UID again until they are successfully removed.

With this commit we'll enumerate the IPC objects currently existing, and
step away from using a UID for the dynamic UID logic if there are any
matching it.

3 files changed, 131 insertions, 41 deletions

diff --git a/src/core/dynamic-user.c b/src/core/dynamic-user.c
index 66e83a74b6..f1b5ee7ecb 100644
--- a/src/core/dynamic-user.c
+++ b/src/core/dynamic-user.c
@@ -21,6 +21,7 @@
 #include <pwd.h>
 #include <sys/file.h>
 
+#include "clean-ipc.h"
 #include "dynamic-user.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -294,7 +295,9 @@ static int pick_uid(char **suggested_paths, const char *name, uid_t *ret_uid) {
                 }
 
                 /* Some superficial check whether this UID/GID might already be taken by some static user */
-                if (getpwuid(candidate) || getgrgid((gid_t) candidate)) {
+                if (getpwuid(candidate) ||
+                    getgrgid((gid_t) candidate) ||
+                    search_ipc(candidate, (gid_t) candidate) != 0) {
                         (void) unlink(lock_path);
                         continue;
                 }
diff --git a/src/shared/clean-ipc.c b/src/shared/clean-ipc.c
index 3e246a1dca..64f81bb5d3 100644
--- a/src/shared/clean-ipc.c
+++ b/src/shared/clean-ipc.c
@@ -54,7 +54,7 @@ static bool match_uid_gid(uid_t subject_uid, gid_t subject_gid, uid_t delete_uid
         return false;
 }
 
-static int clean_sysvipc_shm(uid_t delete_uid, gid_t delete_gid) {
+static int clean_sysvipc_shm(uid_t delete_uid, gid_t delete_gid, bool rm) {
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX];
         bool first = true;
@@ -92,6 +92,9 @@ static int clean_sysvipc_shm(uid_t delete_uid, gid_t delete_gid) {
                 if (!match_uid_gid(uid, gid, delete_uid, delete_gid))
                         continue;
 
+                if (!rm)
+                        return 1;
+
                 if (shmctl(shmid, IPC_RMID, NULL) < 0) {
 
                         /* Ignore entries that are already deleted */
@@ -101,8 +104,11 @@ static int clean_sysvipc_shm(uid_t delete_uid, gid_t delete_gid) {
                         ret = log_warning_errno(errno,
                                                 "Failed to remove SysV shared memory segment %i: %m",
                                                 shmid);
-                } else
+                } else {
                         log_debug("Removed SysV shared memory segment %i.", shmid);
+                        if (ret == 0)
+                                ret = 1;
+                }
         }
 
         return ret;
@@ -111,7 +117,7 @@ fail:
         return log_warning_errno(errno, "Failed to read /proc/sysvipc/shm: %m");
 }
 
-static int clean_sysvipc_sem(uid_t delete_uid, gid_t delete_gid) {
+static int clean_sysvipc_sem(uid_t delete_uid, gid_t delete_gid, bool rm) {
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX];
         bool first = true;
@@ -144,6 +150,9 @@ static int clean_sysvipc_sem(uid_t delete_uid, gid_t delete_gid) {
                 if (!match_uid_gid(uid, gid, delete_uid, delete_gid))
                         continue;
 
+                if (!rm)
+                        return 1;
+
                 if (semctl(semid, 0, IPC_RMID) < 0) {
 
                         /* Ignore entries that are already deleted */
@@ -153,8 +162,11 @@ static int clean_sysvipc_sem(uid_t delete_uid, gid_t delete_gid) {
                         ret = log_warning_errno(errno,
                                                 "Failed to remove SysV semaphores object %i: %m",
                                                 semid);
-                } else
+                } else {
                         log_debug("Removed SysV semaphore %i.", semid);
+                        if (ret == 0)
+                                ret = 1;
+                }
         }
 
         return ret;
@@ -163,7 +175,7 @@ fail:
         return log_warning_errno(errno, "Failed to read /proc/sysvipc/sem: %m");
 }
 
-static int clean_sysvipc_msg(uid_t delete_uid, gid_t delete_gid) {
+static int clean_sysvipc_msg(uid_t delete_uid, gid_t delete_gid, bool rm) {
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX];
         bool first = true;
@@ -197,6 +209,9 @@ static int clean_sysvipc_msg(uid_t delete_uid, gid_t delete_gid) {
                 if (!match_uid_gid(uid, gid, delete_uid, delete_gid))
                         continue;
 
+                if (!rm)
+                        return 1;
+
                 if (msgctl(msgid, IPC_RMID, NULL) < 0) {
 
                         /* Ignore entries that are already deleted */
@@ -206,8 +221,11 @@ static int clean_sysvipc_msg(uid_t delete_uid, gid_t delete_gid) {
                         ret = log_warning_errno(errno,
                                                 "Failed to remove SysV message queue %i: %m",
                                                 msgid);
-                } else
+                } else {
                         log_debug("Removed SysV message queue %i.", msgid);
+                        if (ret == 0)
+                                ret = 1;
+                }
         }
 
         return ret;
@@ -216,7 +234,7 @@ fail:
         return log_warning_errno(errno, "Failed to read /proc/sysvipc/msg: %m");
 }
 
-static int clean_posix_shm_internal(DIR *dir, uid_t uid, gid_t gid) {
+static int clean_posix_shm_internal(DIR *dir, uid_t uid, gid_t gid, bool rm) {
         struct dirent *de;
         int ret = 0, r;
 
@@ -236,9 +254,6 @@ static int clean_posix_shm_internal(DIR *dir, uid_t uid, gid_t gid) {
                         continue;
                 }
 
-                if (!match_uid_gid(st.st_uid, st.st_gid, uid, gid))
-                        continue;
-
                 if (S_ISDIR(st.st_mode)) {
                         _cleanup_closedir_ DIR *kid;
 
@@ -247,29 +262,47 @@ static int clean_posix_shm_internal(DIR *dir, uid_t uid, gid_t gid) {
                                 if (errno != ENOENT)
                                         ret = log_warning_errno(errno, "Failed to enter shared memory directory %s: %m", de->d_name);
                         } else {
-                                r = clean_posix_shm_internal(kid, uid, gid);
+                                r = clean_posix_shm_internal(kid, uid, gid, rm);
                                 if (r < 0)
                                         ret = r;
                         }
 
+                        if (!match_uid_gid(st.st_uid, st.st_gid, uid, gid))
+                                continue;
+
+                        if (!rm)
+                                return 1;
+
                         if (unlinkat(dirfd(dir), de->d_name, AT_REMOVEDIR) < 0) {
 
                                 if (errno == ENOENT)
                                         continue;
 
                                 ret = log_warning_errno(errno, "Failed to remove POSIX shared memory directory %s: %m", de->d_name);
-                        } else
+                        } else {
                                 log_debug("Removed POSIX shared memory directory %s", de->d_name);
+                                if (ret == 0)
+                                        ret = 1;
+                        }
                 } else {
 
+                        if (!match_uid_gid(st.st_uid, st.st_gid, uid, gid))
+                                continue;
+
+                        if (!rm)
+                                return 1;
+
                         if (unlinkat(dirfd(dir), de->d_name, 0) < 0) {
 
                                 if (errno == ENOENT)
                                         continue;
 
                                 ret = log_warning_errno(errno, "Failed to remove POSIX shared memory segment %s: %m", de->d_name);
-                        } else
+                        } else {
                                 log_debug("Removed POSIX shared memory segment %s", de->d_name);
+                                if (ret == 0)
+                                        ret = 1;
+                        }
                 }
         }
 
@@ -279,7 +312,7 @@ fail:
         return log_warning_errno(errno, "Failed to read /dev/shm: %m");
 }
 
-static int clean_posix_shm(uid_t uid, gid_t gid) {
+static int clean_posix_shm(uid_t uid, gid_t gid, bool rm) {
         _cleanup_closedir_ DIR *dir = NULL;
 
         dir = opendir("/dev/shm");
@@ -290,10 +323,10 @@ static int clean_posix_shm(uid_t uid, gid_t gid) {
                 return log_warning_errno(errno, "Failed to open /dev/shm: %m");
         }
 
-        return clean_posix_shm_internal(dir, uid, gid);
+        return clean_posix_shm_internal(dir, uid, gid, rm);
 }
 
-static int clean_posix_mq(uid_t uid, gid_t gid) {
+static int clean_posix_mq(uid_t uid, gid_t gid, bool rm) {
         _cleanup_closedir_ DIR *dir = NULL;
         struct dirent *de;
         int ret = 0;
@@ -326,6 +359,9 @@ static int clean_posix_mq(uid_t uid, gid_t gid) {
                 if (!match_uid_gid(st.st_uid, st.st_gid, uid, gid))
                         continue;
 
+                if (!rm)
+                        return 1;
+
                 fn[0] = '/';
                 strcpy(fn+1, de->d_name);
 
@@ -336,8 +372,11 @@ static int clean_posix_mq(uid_t uid, gid_t gid) {
                         ret = log_warning_errno(errno,
                                                 "Failed to unlink POSIX message queue %s: %m",
                                                 fn);
-                } else
+                } else {
                         log_debug("Removed POSIX message queue %s", fn);
+                        if (ret == 0)
+                                ret = 1;
+                }
         }
 
         return ret;
@@ -346,44 +385,83 @@ fail:
         return log_warning_errno(errno, "Failed to read /dev/mqueue: %m");
 }
 
-int clean_ipc(uid_t uid, gid_t gid) {
+int clean_ipc_internal(uid_t uid, gid_t gid, bool rm) {
         int ret = 0, r;
 
+        /* If 'rm' is true, clean all IPC objects owned by either the specified UID or the specified GID. Return the
+         * last error encountered or == 0 if no matching IPC objects have been found or > 0 if matching IPC objects
+         * have been found and have been removed.
+         *
+         * If 'rm' is false, just search for IPC objects owned by either the specified UID or the specified GID. In
+         * this case we return < 0 on error, > 0 if we found a matching object, == 0 if we didn't.
+         *
+         * As special rule: if UID/GID is specified as root we'll silently not clean up things, and always claim that
+         * there are IPC objects for it. */
+
+        if (uid == 0) {
+                if (!rm)
+                        return 1;
+
+                uid = UID_INVALID;
+        }
+        if (gid == 0) {
+                if (!rm)
+                        return 1;
+
+                gid = GID_INVALID;
+        }
+
         /* Anything to do? */
         if (!uid_is_valid(uid) && !gid_is_valid(gid))
                 return 0;
 
-        /* Refuse to clean IPC of the root user */
-        if (uid == 0 && gid == 0)
-                return 0;
-
-        r = clean_sysvipc_shm(uid, gid);
-        if (r < 0)
-                ret = r;
+        r = clean_sysvipc_shm(uid, gid, rm);
+        if (r != 0) {
+                if (!rm)
+                        return r;
+                if (ret == 0)
+                        ret = r;
+        }
 
-        r = clean_sysvipc_sem(uid, gid);
-        if (r < 0)
-                ret = r;
+        r = clean_sysvipc_sem(uid, gid, rm);
+        if (r != 0) {
+                if (!rm)
+                        return r;
+                if (ret == 0)
+                        ret = r;
+        }
 
-        r = clean_sysvipc_msg(uid, gid);
-        if (r < 0)
-                ret = r;
+        r = clean_sysvipc_msg(uid, gid, rm);
+        if (r != 0) {
+                if (!rm)
+                        return r;
+                if (ret == 0)
+                        ret = r;
+        }
 
-        r = clean_posix_shm(uid, gid);
-        if (r < 0)
-                ret = r;
+        r = clean_posix_shm(uid, gid, rm);
+        if (r != 0) {
+                if (!rm)
+                        return r;
+                if (ret == 0)
+                        ret = r;
+        }
 
-        r = clean_posix_mq(uid, gid);
-        if (r < 0)
-                ret = r;
+        r = clean_posix_mq(uid, gid, rm);
+        if (r != 0) {
+                if (!rm)
+                        return r;
+                if (ret == 0)
+                        ret = r;
+        }
 
         return ret;
 }
 
 int clean_ipc_by_uid(uid_t uid) {
-        return clean_ipc(uid, GID_INVALID);
+        return clean_ipc_internal(uid, GID_INVALID, true);
 }
 
 int clean_ipc_by_gid(gid_t gid) {
-        return clean_ipc(UID_INVALID, gid);
+        return clean_ipc_internal(UID_INVALID, gid, true);
 }
diff --git a/src/shared/clean-ipc.h b/src/shared/clean-ipc.h
index 6ca57f44fd..c04ed3596b 100644
--- a/src/shared/clean-ipc.h
+++ b/src/shared/clean-ipc.h
@@ -21,6 +21,15 @@
 
 #include <sys/types.h>
 
-int clean_ipc(uid_t uid, gid_t gid);
+#include "user-util.h"
+
+int clean_ipc_internal(uid_t uid, gid_t gid, bool rm);
+
+/* Remove all IPC objects owned by the specified UID or GID */
 int clean_ipc_by_uid(uid_t uid);
 int clean_ipc_by_gid(gid_t gid);
+
+/* Check if any IPC object owned by the specified UID or GID exists, returns > 0 if so, == 0 if not */
+static inline int search_ipc(uid_t uid, gid_t gid) {
+        return clean_ipc_internal(uid, gid, false);
+}