Merge pull request #11820 from dm0-/chase

Allow tmpfiles to create files in a root under an unprivileged directory

src/basic/fs-util.c

@@ -926,6 +926,7 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                 if (fstat(child, &st) < 0)
                         return -errno;
                 if ((flags & CHASE_SAFE) &&
+                    (empty_or_root(root) || (size_t)(todo - buffer) > strlen(root)) &&
                     unsafe_transition(&previous_stat, &st))
                         return log_unsafe_transition(fd, child, path, flags);
 

src/test/test-fs-util.c

@@ -154,6 +154,30 @@ static void test_chase_symlinks(void) {
         assert_se(path_equal(result, q));
         result = mfree(result);
 
+        /* Paths underneath the "root" with different UIDs while using CHASE_SAFE */
+
+        if (geteuid() == 0) {
+                p = strjoina(temp, "/user");
+                assert_se(mkdir(p, 0755) >= 0);
+                assert_se(chown(p, UID_NOBODY, GID_NOBODY) >= 0);
+
+                q = strjoina(temp, "/user/root");
+                assert_se(mkdir(q, 0755) >= 0);
+
+                p = strjoina(q, "/link");
+                assert_se(symlink("/", p) >= 0);
+
+                /* Fail when user-owned directories contain root-owned subdirectories. */
+                r = chase_symlinks(p, temp, CHASE_SAFE, &result);
+                assert_se(r == -ENOLINK);
+                result = mfree(result);
+
+                /* Allow this when the user-owned directories are all in the "root". */
+                r = chase_symlinks(p, q, CHASE_SAFE, &result);
+                assert_se(r > 0);
+                result = mfree(result);
+        }
+
         /* Paths using . */
 
         r = chase_symlinks("/etc/./.././", NULL, 0, &result);

src/tmpfiles/tmpfiles.c

@@ -856,7 +856,7 @@ static int path_open_parent_safe(const char *path) {
         if (!dn)
                 return log_oom();
 
-        fd = chase_symlinks(dn, NULL, CHASE_OPEN|CHASE_SAFE|CHASE_WARN, NULL);
+        fd = chase_symlinks(dn, arg_root, CHASE_OPEN|CHASE_SAFE|CHASE_WARN, NULL);
         if (fd < 0 && fd != -ENOLINK)
                 return log_error_errno(fd, "Failed to validate path %s: %m", path);
 
@@ -877,7 +877,7 @@ static int path_open_safe(const char *path) {
                                        "Failed to open invalid path '%s'.",
                                        path);
 
-        fd = chase_symlinks(path, NULL, CHASE_OPEN|CHASE_SAFE|CHASE_WARN|CHASE_NOFOLLOW, NULL);
+        fd = chase_symlinks(path, arg_root, CHASE_OPEN|CHASE_SAFE|CHASE_WARN|CHASE_NOFOLLOW, NULL);
         if (fd < 0 && fd != -ENOLINK)
                 return log_error_errno(fd, "Failed to validate path %s: %m", path);
 
@@ -2256,7 +2256,7 @@ static int process_item(Item *i, OperationMask operation) {
 
         i->done |= operation;
 
-        r = chase_symlinks(i->path, NULL, CHASE_NO_AUTOFS|CHASE_WARN, NULL);
+        r = chase_symlinks(i->path, arg_root, CHASE_NO_AUTOFS|CHASE_WARN, NULL);
         if (r == -EREMOTE) {
                 log_notice_errno(r, "Skipping %s", i->path);
                 return 0;

test/TEST-22-TMPFILES/test-08.sh

@@ -0,0 +1,32 @@
+#! /bin/bash
+#
+# Verify tmpfiles can run in a root directory under a path prefix that contains
+# directories owned by unprivileged users, for example when a root file system
+# is mounted in a regular user's home directory.
+#
+# https://github.com/systemd/systemd/pull/11820
+#
+
+set -e
+
+rm -fr /tmp/root /tmp/user
+mkdir -p /tmp/root /tmp/user/root
+chown daemon:daemon /tmp/user
+
+# Verify the command works as expected with no prefix or a root-owned prefix.
+echo 'd /tmp/root/test1' | systemd-tmpfiles --create -
+test -d /tmp/root/test1
+echo 'd /test2' | systemd-tmpfiles --root=/tmp/root --create -
+test -d /tmp/root/test2
+
+# Verify the command fails to write to a root-owned subdirectory under an
+# unprivileged user's directory when it's not part of the prefix, as expected
+# by the unsafe_transition function.
+! echo 'd /tmp/user/root/test' | systemd-tmpfiles --create -
+! test -e /tmp/user/root/test
+! echo 'd /user/root/test' | systemd-tmpfiles --root=/tmp --create -
+! test -e /tmp/user/root/test
+
+# Verify the above works when all user-owned directories are in the prefix.
+echo 'd /test' | systemd-tmpfiles --root=/tmp/user/root --create -
+test -d /tmp/user/root/test