Commit 613cc2b6 authored Dec 21, 2016 by Aleksa Sarai Committed by Al Viro Dec 22, 2016

fs: exec: apply CLOEXEC before changing dumpable task flags



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@opencontainers.org
Cc: <stable@vger.kernel.org> # v3.2+
Reported-by: Michael Crosby <crosbymichael@gmail.com>
Signed-off-by: Aleksa Sarai <asarai@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent e522751d

fs/exec.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -1268,6 +1268,13 @@ int flush_old_exec(struct linux_binprm * bprm)
		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:
		@@ -1330,7 +1337,6 @@ void setup_new_exec(struct linux_binprm * bprm)
		group */
		current->self_exec_id++;
		flush_signal_handlers(current, 0);
		do_close_on_exec(current->files);
		}
		EXPORT_SYMBOL(setup_new_exec);

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