ftpd: escape chroot prior to re-executing ls helper

When we merely chdir to saved "real" root fd,
exec("proc/self/exe") works for static executables but not
for dynamic ones (they can't find their interpreter).

With this patch, we also *chroot* to real root.
As a bonus, this gives us proper usernames, timezone conversion
etc.

function                                             old     new   delta
popen_ls                                             203     259     +56
ftpd_main                                           2362    2366      +4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 21 insertions, 32 deletions

diff --git a/networking/ftpd.c b/networking/ftpd.c
index e724734df..839a85d73 100644
--- a/networking/ftpd.c
+++ b/networking/ftpd.c
@@ -620,16 +620,12 @@ popen_ls(const char *opt)
         const char *argv[5];
         struct fd_pair outfd;
         pid_t pid;
-
+#if !BB_MMU
+        int cur_fd = xopen(".", O_RDONLY | O_DIRECTORY);
+#endif
         argv[0] = "ftpd";
         argv[1] = opt; /* "-l" or "-1" */
-#if BB_MMU
         argv[2] = "--";
-#else
-        /* NOMMU ftpd ls helper chdirs to argv[2],
-         * preventing peer from seeing real root. */
-        argv[2] = xrealloc_getcwd_or_warn(NULL);
-#endif
         argv[3] = G.ftp_arg;
         argv[4] = NULL;
 
@@ -651,16 +647,6 @@ popen_ls(const char *opt)
         pid = BB_MMU ? xfork() : xvfork();
         if (pid == 0) {
                 /* child */
-#if !BB_MMU
-                /* On NOMMU, we want to execute a child - copy of ourself.
-                 * In chroot we usually can't do it. Thus we chdir
-                 * out of the chroot back to original root,
-                 * and (see later below) execute bb_busybox_exec_path
-                 * relative to current directory */
-                if (fchdir(G.root_fd) != 0)
-                        _exit(127);
-                /*close(G.root_fd); - close_on_exec_on() took care of this */
-#endif
                 /* NB: close _first_, then move fd! */
                 close(outfd.rd);
                 xmove_fd(outfd.wr, STDOUT_FILENO);
@@ -674,19 +660,25 @@ popen_ls(const char *opt)
                 /* memset(&G, 0, sizeof(G)); - ls_main does it */
                 exit(ls_main(ARRAY_SIZE(argv) - 1, (char**) argv));
 #else
-                /* + 1: we must use relative path here if in chroot.
-                 * For example, execv("/proc/self/exe") will fail, since
-                 * it looks for "/proc/self/exe" _relative to chroot!_ */
-                execv(bb_busybox_exec_path + 1, (char**) argv);
+                /* On NOMMU, we want to execute a child - copy of ourself
+                 * in order to unblock parent after vfork.
+                 * In chroot we usually can't re-exec. Thus we escape
+                 * out of the chroot back to original root.
+                 */
+                if (G.root_fd >= 0) {
+                        if (fchdir(G.root_fd) != 0 || chroot(".") != 0)
+                                _exit(127);
+                        /*close(G.root_fd); - close_on_exec_on() took care of this */
+                }
+                /* Child expects directory to list on fd #3 */
+                xmove_fd(cur_fd, 3);
+                execv(bb_busybox_exec_path, (char**) argv);
                 _exit(127);
 #endif
         }
 
         /* parent */
         close(outfd.wr);
-#if !BB_MMU
-        free((char*)argv[2]);
-#endif
         return outfd.rd;
 }
 
@@ -705,8 +697,6 @@ handle_dir_common(int opts)
         if (!(opts & USE_CTRL_CONN) && !port_or_pasv_was_seen())
                 return; /* port_or_pasv_was_seen emitted error response */
 
-        /* -n prevents user/groupname display,
-         * which can be problematic in chroot */
         ls_fd = popen_ls((opts & LONG_LISTING) ? "-l" : "-1");
         ls_fp = xfdopen_for_read(ls_fd);
 /* FIXME: filenames with embedded newlines are mishandled */
@@ -1139,11 +1129,10 @@ int ftpd_main(int argc UNUSED_PARAM, char **argv)
         opts = getopt32(argv, "l1vS" IF_FEATURE_FTP_WRITE("w") "t:T:", &G.timeout, &abs_timeout, &G.verbose, &verbose_S);
         if (opts & (OPT_l|OPT_1)) {
                 /* Our secret backdoor to ls */
-/* TODO: pass -n? It prevents user/group resolution, which may not work in chroot anyway */
 /* TODO: pass -A? It shows dot files */
 /* TODO: pass --group-directories-first? would be nice, but ls doesn't do that yet */
-                xchdir(argv[2]);
-                argv[2] = (char*)"--";
+                if (fchdir(3) != 0)
+                        _exit(127);
                 /* memset(&G, 0, sizeof(G)); - ls_main does it */
                 return ls_main(argc, argv);
         }
@@ -1185,9 +1174,9 @@ int ftpd_main(int argc UNUSED_PARAM, char **argv)
         G.root_fd = xopen("/", O_RDONLY | O_DIRECTORY);
         close_on_exec_on(G.root_fd);
 #endif
-
-        if (argv[optind]) {
-                xchroot(argv[optind]);
+        argv += optind;
+        if (argv[0]) {
+                xchroot(argv[0]);
         }
 
         //umask(077); - admin can set umask before starting us