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>

diff --git a/networking/ftpd.c b/networking/ftpd.c
index e724734dfcd1f137f26cc3ba80c94c7a22c8072e..839a85d73da273dc8bb4f9017961bb7efdcb31f7 100644 (file)
--- 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