[ PATCH] execve: must clear current->clear_child_tid

[Eric Dumazet <eric.dumazet@gmail.com>]

While looking at Jens Rosenboom bug report about strange sys_futex call done
from a dying "ps" program, we found following problem.

clone() syscall has special support for TID of created threads.
This support includes two features.

One (CLONE_CHILD_SETTID) is to set an integer into user memory
with the TID value.

One (CLONE_CHILD_CLEARTID) is to clear this same integer once
the created thread dies.

The integer location is a user provided pointer, provided at clone()
time.

kernel keeps this pointer value into current->clear_child_tid.

At execve() time, we should make sure kernel doesnt keep
this user provided pointer, as full user memory is replaced by a new one.

As glibc fork() actually uses clone() syscall with CLONE_CHILD_SETTID
and CLONE_CHILD_CLEARTID set, chances are high that we might corrupt
user memory in forked processes.

Following sequence could happen:

1) bash (or any program) starts a new process, by a fork() call
that glibc maps to a clone( ... CLONE_CHILD_SETTID
| CLONE_CHILD_CLEARTID ...) syscall

2) When new process starts, its current->clear_child_tid is set to a location
that has a meaning only in bash (or initial program) context  (&THREAD_SELF->tid)

3) This new process does the execve() syscall to start a new program.
   current->clear_child_tid is left unchanged (a non NULL value)

4) If this new program creates some threads, and initial thread exits,
kernel will attempt to clear the integer pointed by current->clear_child_tid
from mm_release() :

        if (tsk->clear_child_tid
            && !(tsk->flags & PF_SIGNALED)
            && atomic_read(&mm->mm_users) > 1) {
                u32 __user * tidptr = tsk->clear_child_tid;
                tsk->clear_child_tid = NULL;

                /*
                 * We don't check the error code - if userspace has
                 * not set up a proper pointer then tough luck.
                 */
<< here >>      put_user(0, tidptr);
                sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
        }

5) OR : if new program is not multi-threaded, but spied by /proc/pid users
(ps command for example), mm_users > 1, and the exiting program could
corrupt 4 bytes in a persistent memory area (shm or memory mapped file)

If current->clear_child_tid points to a writeable portion of memory
of the new program, kernel happily and silently corrupts 4 bytes of memory,
with unexpected effects.

Fix is straightforward and should not break any sane program.

Reported-by: Jens Rosenboom <jens@mcbone.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Jens Rosenboom <jens@mcbone.net>
---
 fs/compat.c |    1 +
 fs/exec.c   |    1 +
 2 files changed, 2 insertions(+)

diff --git a/fs/compat.c b/fs/compat.c
index 94502da..deb1049 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -1550,6 +1550,7 @@ int compat_do_execve(char * filename,
 	mutex_unlock(&current->cred_guard_mutex);
 	acct_update_integrals(current);
 	free_bprm(bprm);
+	current->clear_child_tid = NULL;
 	if (displaced)
 		put_files_struct(displaced);
 	return retval;
diff --git a/fs/exec.c b/fs/exec.c
index 4a8849e..e275652 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1343,6 +1343,7 @@ int do_execve(char * filename,
 	mutex_unlock(&current->cred_guard_mutex);
 	acct_update_integrals(current);
 	free_bprm(bprm);
+	current->clear_child_tid = NULL;
 	if (displaced)
 		put_files_struct(displaced);
 	return retval;