execute: Chown credentials files and directories to service group

For a userns root user to be able to access the credentials, both
the uid and gid of the credentials directory have to be mapped into
the userns. Currently, the credentials directory group is root, which
we obviously do not want to map in to a userns, so let's make sure
that the credentials directory and files are owned by the service
group instead, which can generally be safely mapped into the userns.

Since we use permissions mode 0600, this shouldn't cause any change
in who is able to access the credentials.

Fixes #28747

(cherry picked from commit 3a78b0e)

Author: DaanDeMeyer

src/core/execute.c

@@ -2746,6 +2746,7 @@ static int write_credential(
                 const void *data,
                 size_t size,
                 uid_t uid,
+                gid_t gid,
                 bool ownership_ok) {
 
         _cleanup_(unlink_and_freep) char *tmp = NULL;
@@ -2783,7 +2784,7 @@ static int write_credential(
                                             * user can no longer chmod() the file to gain write access. */
                                 return r;
 
-                        if (fchown(fd, uid, GID_INVALID) < 0)
+                        if (fchown(fd, uid, gid) < 0)
                                 return -errno;
                 }
         }
@@ -2845,6 +2846,7 @@ static int maybe_decrypt_and_write_credential(
                 const char *id,
                 bool encrypted,
                 uid_t uid,
+                gid_t gid,
                 bool ownership_ok,
                 const char *data,
                 size_t size,
@@ -2870,7 +2872,7 @@ static int maybe_decrypt_and_write_credential(
         if (add > *left)
                 return -E2BIG;
 
-        r = write_credential(dir_fd, id, data, size, uid, ownership_ok);
+        r = write_credential(dir_fd, id, data, size, uid, gid, ownership_ok);
         if (r < 0)
                 return log_debug_errno(r, "Failed to write credential '%s': %m", id);
 
@@ -2885,6 +2887,7 @@ static int load_credential_glob(
                 ReadFullFileFlags flags,
                 int write_dfd,
                 uid_t uid,
+                gid_t gid,
                 bool ownership_ok,
                 uint64_t *left) {
 
@@ -2932,6 +2935,7 @@ static int load_credential_glob(
                                 fn,
                                 encrypted,
                                 uid,
+                                gid,
                                 ownership_ok,
                                 data, size,
                                 left);
@@ -2955,6 +2959,7 @@ static int load_credential(
                 int read_dfd,
                 int write_dfd,
                 uid_t uid,
+                gid_t gid,
                 bool ownership_ok,
                 uint64_t *left) {
 
@@ -3066,7 +3071,7 @@ static int load_credential(
         if (r < 0)
                 return log_debug_errno(r, "Failed to read credential '%s': %m", path);
 
-        return maybe_decrypt_and_write_credential(write_dfd, id, encrypted, uid, ownership_ok, data, size, left);
+        return maybe_decrypt_and_write_credential(write_dfd, id, encrypted, uid, gid, ownership_ok, data, size, left);
 }
 
 struct load_cred_args {
@@ -3076,6 +3081,7 @@ struct load_cred_args {
         const char *unit;
         int dfd;
         uid_t uid;
+        gid_t gid;
         bool ownership_ok;
         uint64_t *left;
 };
@@ -3123,6 +3129,7 @@ static int load_cred_recurse_dir_cb(
                         dir_fd,
                         args->dfd,
                         args->uid,
+                        args->gid,
                         args->ownership_ok,
                         args->left);
         if (r < 0)
@@ -3137,6 +3144,7 @@ static int acquire_credentials(
                 const char *unit,
                 const char *p,
                 uid_t uid,
+                gid_t gid,
                 bool ownership_ok) {
 
         uint64_t left = CREDENTIALS_TOTAL_SIZE_MAX;
@@ -3186,6 +3194,7 @@ static int acquire_credentials(
                                         AT_FDCWD,
                                         dfd,
                                         uid,
+                                        gid,
                                         ownership_ok,
                                         &left);
                 else
@@ -3204,6 +3213,7 @@ static int acquire_credentials(
                                                 .unit = unit,
                                                 .dfd = dfd,
                                                 .uid = uid,
+                                                .gid = gid,
                                                 .ownership_ok = ownership_ok,
                                                 .left = &left,
                                         });
@@ -3227,6 +3237,7 @@ static int acquire_credentials(
                                 READ_FULL_FILE_SECURE|READ_FULL_FILE_FAIL_WHEN_LARGER,
                                 dfd,
                                 uid,
+                                gid,
                                 ownership_ok,
                                 &left);
                 if (r < 0)
@@ -3244,6 +3255,7 @@ static int acquire_credentials(
                                 READ_FULL_FILE_SECURE|READ_FULL_FILE_FAIL_WHEN_LARGER|READ_FULL_FILE_UNBASE64,
                                 dfd,
                                 uid,
+                                gid,
                                 ownership_ok,
                                 &left);
                 if (r < 0)
@@ -3281,7 +3293,7 @@ static int acquire_credentials(
                 if (add > left)
                         return -E2BIG;
 
-                r = write_credential(dfd, sc->id, data, size, uid, ownership_ok);
+                r = write_credential(dfd, sc->id, data, size, uid, gid, ownership_ok);
                 if (r < 0)
                         return r;
 
@@ -3304,7 +3316,7 @@ static int acquire_credentials(
                         if (!ownership_ok)
                                 return r;
 
-                        if (fchown(dfd, uid, GID_INVALID) < 0)
+                        if (fchown(dfd, uid, gid) < 0)
                                 return -errno;
                 }
         }
@@ -3320,7 +3332,8 @@ static int setup_credentials_internal(
                 const char *workspace,    /* This is where we can prepare it before moving it to the final place */
                 bool reuse_workspace,     /* Whether to reuse any existing workspace mount if it already is a mount */
                 bool must_mount,          /* Whether to require that we mount something, it's not OK to use the plain directory fall back */
-                uid_t uid) {
+                uid_t uid,
+                gid_t gid) {
 
         int r, workspace_mounted; /* negative if we don't know yet whether we have/can mount something; true
                                    * if we mounted something; false if we definitely can't mount anything */
@@ -3406,7 +3419,7 @@ static int setup_credentials_internal(
 
         (void) label_fix_full(AT_FDCWD, where, final, 0);
 
-        r = acquire_credentials(context, params, unit, where, uid, workspace_mounted);
+        r = acquire_credentials(context, params, unit, where, uid, gid, workspace_mounted);
         if (r < 0)
                 return r;
 
@@ -3461,7 +3474,8 @@ static int setup_credentials(
                 const ExecContext *context,
                 const ExecParameters *params,
                 const char *unit,
-                uid_t uid) {
+                uid_t uid,
+                gid_t gid) {
 
         _cleanup_free_ char *p = NULL, *q = NULL;
         int r;
@@ -3528,7 +3542,8 @@ static int setup_credentials(
                                 u,       /* temporary workspace to overmount */
                                 true,    /* reuse the workspace if it is already a mount */
                                 false,   /* it's OK to fall back to a plain directory if we can't mount anything */
-                                uid);
+                                uid,
+                                gid);
 
                 (void) rmdir(u); /* remove the workspace again if we can. */
 
@@ -3566,7 +3581,8 @@ static int setup_credentials(
                                 "/dev/shm",  /* temporary workspace to overmount */
                                 false,       /* do not reuse /dev/shm if it is already a mount, under no circumstances */
                                 true,        /* insist that something is mounted, do not allow fallback to plain directory */
-                                uid);
+                                uid,
+                                gid);
                 if (r < 0)
                         goto child_fail;
 
@@ -5295,7 +5311,7 @@ static int exec_child(
         }
 
         if (FLAGS_SET(params->flags, EXEC_WRITE_CREDENTIALS)) {
-                r = setup_credentials(context, params, unit->id, uid);
+                r = setup_credentials(context, params, unit->id, uid, gid);
                 if (r < 0) {
                         *exit_status = EXIT_CREDENTIALS;
                         return log_unit_error_errno(unit, r, "Failed to set up credentials: %m");