[PATCH v6] seunshare: fix the frail tmpdir cleanup

[Rahul Sandhu <nvraxn@gmail.com>]

For some reason, rm is invoked via system (3) to cleanup the runtime
temp directory.  This really isn't all that robust, *especially* given
that seunshare is supposed to be a security boundary.  Instead do this
using libc, the API designed to be used within C programs.

Also make sure that we don't follow symbolic links; the input being
deleted is untrusted, and hence a malicious symbolic link may be placed
outside of the sandbox.

Signed-off-by: Rahul Sandhu <nvraxn@gmail.com>
---
 sandbox/seunshare.c | 77 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 65 insertions(+), 12 deletions(-)

v2: don't use else after return
v3: don't follow symlinks in rm_rf ().  This is pretty important as we
    we are operating on an untrusted directory, which may have symlinks
    pointed to privileged content.  However, as we only really need to
    operate on the contents of the tmpdir, we can ignore symlinks.
v4: fix spelling in commit message
v5: fix spelling in comment
v6: fix the error checking for the rm_rf () function.

diff --git a/sandbox/seunshare.c b/sandbox/seunshare.c
index 106f625f..337a9205 100644
--- a/sandbox/seunshare.c
+++ b/sandbox/seunshare.c
@@ -403,6 +403,66 @@ err:
 	return rc;
 }
 
+/*
+ * Recursively delete a directory.
+ * SAFETY: This function will NOT follow symbolic links (AT_SYMLINK_NOFOLLOW).
+ *         As a result, this function can be run safely on a directory owned by
+ *         a non-root user: symbolic links to root paths (such as /root) will
+ *         not be followed.
+ */
+static bool rm_rf(int targetfd, const char *path) {
+	struct stat statbuf;
+
+	if (fstatat(targetfd, path, &statbuf, AT_SYMLINK_NOFOLLOW) < 0) {
+		if (errno == ENOENT) {
+			return true;
+		}
+		perror("fstatat");
+		return false;
+	}
+
+	if (S_ISDIR(statbuf.st_mode)) {
+		const int newfd = openat(targetfd, path, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+		if (newfd < 0) {
+			perror("openat");
+			return false;
+		}
+
+		DIR *dir = fdopendir(newfd);
+		if (!dir) {
+			perror("fdopendir");
+			close(newfd);
+			return false;
+		}
+
+		struct dirent *entry;
+		int rc = true;
+		while ((entry = readdir(dir)) != NULL) {
+			if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
+				continue;
+			}
+
+			if (!rm_rf(dirfd(dir), entry->d_name)) {
+				rc = false;
+			}
+		}
+
+		closedir(dir);
+
+		if (unlinkat(targetfd, path, AT_REMOVEDIR) < 0) {
+			perror("unlinkat");
+			rc = false;
+		}
+
+		return rc;
+	}
+	if (unlinkat(targetfd, path, 0) < 0) {
+		perror("unlinkat");
+		return false;
+	}
+	return true;
+}
+
 /**
  * Clean up runtime temporary directory.  Returns 0 if no problem was detected,
  * >0 if some error was detected, but errors here are treated as non-fatal and
@@ -428,24 +488,17 @@ static int cleanup_tmpdir(const char *tmpdir, const char *src,
 		free(cmdbuf); cmdbuf = NULL;
 	}
 
-	/* remove files from the runtime temporary directory */
-	if (asprintf(&cmdbuf, "/bin/rm -r '%s/' 2>/dev/null", tmpdir) == -1) {
-		fprintf(stderr, _("Out of memory\n"));
-		cmdbuf = NULL;
+	if ((uid_t)setfsuid(0) != 0) {
+		/* setfsuid does not return error, but this check makes code checkers happy */
 		rc++;
 	}
-	/* this may fail if there's root-owned file left in the runtime tmpdir */
-	if (cmdbuf && spawn_command(cmdbuf, pwd->pw_uid) != 0) rc++;
-	free(cmdbuf); cmdbuf = NULL;
 
-	/* remove runtime temporary directory */
-	if ((uid_t)setfsuid(0) != 0) {
-		/* setfsuid does not return error, but this check makes code checkers happy */
+	/* Recursively remove the runtime temp directory.  */
+	if (!rm_rf(AT_FDCWD, tmpdir)) {
+		fprintf(stderr, _("Failed to recursively remove directory %s\n"), tmpdir);
 		rc++;
 	}
 
-	if (pwd->pw_uid != 0 && rmdir(tmpdir) == -1)
-		fprintf(stderr, _("Failed to remove directory %s: %s\n"), tmpdir, strerror(errno));
 	if ((uid_t)setfsuid(pwd->pw_uid) != 0) {
 		fprintf(stderr, _("unable to switch back to user after clearing tmp dir\n"));
 		rc++;
-- 
2.50.1