Patch "fs: exec: apply CLOEXEC before changing dumpable task flags" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

fs: exec: apply CLOEXEC before changing dumpable task flags

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fs-exec-apply-cloexec-before-changing-dumpable-task-flags.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Aleksa Sarai <[email protected]>
Date: Wed, 21 Dec 2016 16:26:24 +1100
Subject: fs: exec: apply CLOEXEC before changing dumpable task flags

From: Aleksa Sarai <[email protected]>

commit 613cc2b6f272c1a8ad33aefa21cad77af23139f7 upstream.

If you have a process that has set itself to be non-dumpable, and it then undergoes exec(2), any CLOEXEC file descriptors it has open are "exposed" during a race window between the dumpable flags of the process being reset for exec(2) and CLOEXEC being applied to the file descriptors. This can be exploited by a process by attempting to access /proc/<pid>/fd/... during this window, without requiring CAP_SYS_PTRACE.

The race in question is after set_dumpable has been (for get_link, though the trace is basically the same for readlink):

[vfs]
-> proc_pid_link_inode_operations.get_link
-> proc_pid_get_link

      -> proc_fd_access_allowed
         -> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);

Which will return 0, during the race window and CLOEXEC file descriptors will still be open during this window because do_close_on_exec has not been called yet. As a result, the ordering of these calls should be reversed to avoid this race window.

This is of particular concern to container runtimes, where joining a PID namespace with file descriptors referring to the host filesystem can result in security issues (since PRCTL_SET_DUMPABLE doesn't protect against access of CLOEXEC file descriptors -- file descriptors which may reference filesystem objects the container shouldn't have access to).

---
 fs/exec.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

@@ -19,7 +19,7 @@
  * current->executable is only used by the procfs.  This allows a dispatch
  * table to check for several different types  of binary formats.  We keep
  * trying until we recognize the file or we run out of supported binary
- * formats. 
+ * formats.
  */
 
 #include <linux/slab.h>

@@ -1261,6 +1261,13 @@ int flush_old_exec(struct linux_binprm *
 	flush_thread();
 	current->personality &= ~bprm->per_clear;
 
+	/*
+	 * We have to apply CLOEXEC before we change whether the process is
+	 * dumpable (in setup_new_exec) to avoid a race with a process in userspace
+	 * trying to access the should-be-closed file descriptors of a process
+	 * undergoing exec(2).
+	 */
+	do_close_on_exec(current->files);
 	return 0;
 
 out:

@@ -1323,7 +1330,6 @@ void setup_new_exec(struct linux_binprm
 	   group */
 	current->self_exec_id++;
 	flush_signal_handlers(current, 0);
-	do_close_on_exec(current->files);
 }
 EXPORT_SYMBOL(setup_new_exec);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/fs-exec-apply-cloexec-before-changing-dumpable-task-flags.patch