[ 090/110] pidns: fix vfork() after unshare(CLONE_NEWPID)

From: Greg Kroah-Hartman
Date: Tue Sep 24 2013 - 20:56:02 EST

3.10-stable review patch. If anyone has any objections, please let me know.


From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit e79f525e99b04390ca4d2366309545a836c03bf1 upstream.

Commit 8382fcac1b81 ("pidns: Outlaw thread creation after
unshare(CLONE_NEWPID)") nacks CLONE_VM if the forking process unshared
pid_ns, this obviously breaks vfork:

int main(void)
assert(unshare(CLONE_NEWUSER | CLONE_NEWPID) == 0);
assert(vfork() >= 0);
return 0;

fails without this patch.

Change this check to use CLONE_SIGHAND instead. This also forbids
CLONE_THREAD automatically, and this is what the comment implies.

We could probably even drop CLONE_SIGHAND and use CLONE_THREAD, but it
would be safer to not do this. The current check denies CLONE_SIGHAND
implicitely and there is no reason to change this.

Eric said "CLONE_SIGHAND is fine. CLONE_THREAD would be even better.
Having shared signal handling between two different pid namespaces is
the case that we are fundamentally guarding against."

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reported-by: Colin Walters <walters@xxxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/fork.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1171,10 +1171,11 @@ static struct task_struct *copy_process(
return ERR_PTR(-EINVAL);

- * If the new process will be in a different pid namespace
- * don't allow the creation of threads.
+ * If the new process will be in a different pid namespace don't
+ * allow it to share a thread group or signal handlers with the
+ * forking task.
- if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
+ if ((clone_flags & (CLONE_SIGHAND | CLONE_NEWPID)) &&
(task_active_pid_ns(current) != current->nsproxy->pid_ns))
return ERR_PTR(-EINVAL);

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