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

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Date:	Thu, 06 Aug 2015 16:16:44 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Ricky Zhou <rickyz@...omium.org>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness

Oleg Nesterov <oleg@...hat.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)
--
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