linux-kernel - [PATCH 3.16 121/370] fs: exec: apply CLOEXEC before changing dumpable task flags

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Date:   Fri, 10 Mar 2017 11:46:22 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Aleksa Sarai" <asarai@...e.de>,
        "Al Viro" <viro@...iv.linux.org.uk>, dev@...ncontainers.org,
        "Michael Crosby" <crosbymichael@...il.com>
Subject: [PATCH 3.16 121/370] fs: exec: apply CLOEXEC before changing
 dumpable task flags

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Aleksa Sarai <asarai@...e.de>

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).

Cc: dev@...ncontainers.org
Reported-by: Michael Crosby <crosbymichael@...il.com>
Signed-off-by: Aleksa Sarai <asarai@...e.de>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/exec.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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>
@@ -1087,6 +1087,13 @@ 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:
@@ -1136,7 +1143,6 @@ 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);