Re: [PATCH] user_ns: use correct check for single-threadedness

[Eric W. Biederman]

Oleg Nesterov <o...@redhat.com> writes:

> On 08/05, Eric W. Biederman wrote:
>>
>> So I have to ask.
>
> I hope you are asking someone else, not me ;) I never understood what
> exactly we try to restrict and why.

I think I was just asking rhetorically, and asking myself.

>> Is it possible to rework these checks such that we
>> look at the sighand struct and signal sharing handling sharing instead
>> of the count on the mm_struct?
>
> Then why we can't simply check thread_group_empty() == T ? Why should we
> worry about CLONE_SIGHAND at all?

<rant> 
thread_group_empty() for a thread group with a single member is a
confusing name.
 </rant>

CLONE_SIGHAND is just a hair excessive, you have to at least look at
your per thread register to see which namespace to interpret the values
of the signals in.  I suppose that is confusing but not totally fatal.

Without changing the semantics, just correcting the implementation
of the code we have now this is the code I get:

diff --git a/kernel/fork.c b/kernel/fork.c
index 1bfefc6f96a4..c95757c15fcb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1866,13 +1866,13 @@ static int check_unshare_flags(unsigned long 
unshare_flags)
                                CLONE_NEWUSER|CLONE_NEWPID))
                return -EINVAL;
        /*
-        * Not implemented, but pretend it works if there is nothing to
-        * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
-        * needs to unshare vm.
+        * Not implemented, but pretend it works if there is nothing
+        * to unshare.  Note that unsharing the address space or the
+        * signal handlers also need to unshare the signal queues (aka
+        * CLONE_THREAD).
         */
-       if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
-               /* FIXME: get_task_mm() increments ->mm_users */
-               if (atomic_read(&current->mm->mm_users) > 1)
+       if (unshare_flags & CLONE_THREAD) {
+               if (!thread_group_empty(current))
                        return -EINVAL;
        }
 
@@ -1936,21 +1936,23 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
        int err;
 
        /*
-        * If unsharing a user namespace must also unshare the thread.
+        * If unsharing a user namespace must also unshare the signal
+        * handlers and unshare the filesystem root and working
+        * directories.
         */
        if (unshare_flags & CLONE_NEWUSER)
-               unshare_flags |= CLONE_THREAD | CLONE_FS;
-       /*
-        * If unsharing a thread from a thread group, must also unshare vm.
-        */
-       if (unshare_flags & CLONE_THREAD)
-               unshare_flags |= CLONE_VM;
+               unshare_flags |= CLONE_SIGHAND | CLONE_FS;
        /*
         * If unsharing vm, must also unshare signal handlers.
         */
        if (unshare_flags & CLONE_VM)
                unshare_flags |= CLONE_SIGHAND;
        /*
+        * If unsharing a signal handlers, must also unshare the signal queues.
+        */
+       if (unshare_flags & CLONE_SIGHAND)
+               unshare_flags |= CLONE_THREAD;
+       /*
         * If unsharing namespace, must also unshare filesystem information.
         */
        if (unshare_flags & CLONE_NEWNS)
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 4109f8320684..45a5cbf97715 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -976,8 +976,8 @@ static int userns_install(struct nsproxy *nsproxy, struct 
ns_common *ns)
        if (user_ns == current_user_ns())
                return -EINVAL;
 
-       /* Threaded processes may not enter a different user namespace */
-       if (atomic_read(&current->mm->mm_users) > 1)
+       /* Shared signal handlers must live in the same user namespace */
+       if (atomic_read(&current->sighand->count) > 1)
                return -EINVAL;
 
        if (current->fs->users != 1)
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/