exec: use -ELOOP for max recursion depth

To avoid an explosion of request_module calls on a chain of abusive
scripts, fail maximum recursion with -ELOOP instead of -ENOEXEC. As soon
as maximum recursion depth is hit, the error will fail all the way back
up the chain, aborting immediately.

This also has the side-effect of stopping the user's shell from attempting
to reexecute the top-level file as a shell script. As seen in the
dash source:

        if (cmd != path_bshell && errno == ENOEXEC) {
                *argv-- = cmd;
                *argv = cmd = path_bshell;
                goto repeat;
        }

The above logic was designed for running scripts automatically that lacked
the "#!" header, not to re-try failed recursion. On a legitimate -ENOEXEC,
things continue to behave as the shell expects.

Additionally, when tracking recursion, the binfmt handlers should not be
involved. The recursion being tracked is the depth of calls through
search_binary_handler(), so that function should be exclusively responsible
for tracking the depth.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: halfdog <me@halfdog.net>
Cc: P J P <ppandit@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 721a29929511..d5eb9e605ffd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1356,6 +1356,10 @@ int search_binary_handler(struct linux_binprm *bprm)
         struct linux_binfmt *fmt;
         pid_t old_pid, old_vpid;
 
+        /* This allows 4 levels of binfmt rewrites before failing hard. */
+        if (depth > 5)
+                return -ELOOP;
+
         retval = security_bprm_check(bprm);
         if (retval)
                 return retval;
@@ -1380,12 +1384,8 @@ int search_binary_handler(struct linux_binprm *bprm)
                         if (!try_module_get(fmt->module))
                                 continue;
                         read_unlock(&binfmt_lock);
+                        bprm->recursion_depth = depth + 1;
                         retval = fn(bprm);
-                        /*
-                         * Restore the depth counter to its starting value
-                         * in this call, so we don't have to rely on every
-                         * load_binary function to restore it on return.
-                         */
                         bprm->recursion_depth = depth;
                         if (retval >= 0) {
                                 if (depth == 0) {