[added to the 3.18 stable tree] unshare: Unsharing a thread does not require unsharing a vm

[Sasha Levin]

From: "Eric W. Biederman" <ebied...@xmission.com>

This patch has been added to the 3.18 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 12c641ab8270f787dfcce08b5f20ce8b65008096 ]

In the logic in the initial commit of unshare made creating a new
thread group for a process, contingent upon creating a new memory
address space for that process.  That is wrong.  Two separate
processes in different thread groups can share a memory address space
and clone allows creation of such proceses.

This is significant because it was observed that mm_users > 1 does not
mean that a process is multi-threaded, as reading /proc/PID/maps
temporarily increments mm_users, which allows other processes to
(accidentally) interfere with unshare() calls.

Correct the check in check_unshare_flags() to test for
!thread_group_empty() for CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM.
For sighand->count > 1 for CLONE_SIGHAND and CLONE_VM.
For !current_is_single_threaded instead of mm_users > 1 for CLONE_VM.

By using the correct checks in unshare this removes the possibility of
an accidental denial of service attack.

Additionally using the correct checks in unshare ensures that only an
explicit unshare(CLONE_VM) can possibly trigger the slow path of
current_is_single_threaded().  As an explict unshare(CLONE_VM) is
pointless it is not expected there are many applications that make
that call.

Cc: stable@vger.kernel.org
Fixes: b2e0d98705e60e45bbb3c0032c48824ad7ae0704 userns: Implement unshare of 
the user namespace
Reported-by: Ricky Zhou <ric...@chromium.org>
Reported-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: Kees Cook <keesc...@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
Signed-off-by: Sasha Levin <sasha.le...@oracle.com>
---
 kernel/fork.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 9b7d746..0a4f601 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1800,13 +1800,21 @@ 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 (!thread_group_empty(current))
+                       return -EINVAL;
+       }
+       if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
+               if (atomic_read(&current->sighand->count) > 1)
+                       return -EINVAL;
+       }
+       if (unshare_flags & CLONE_VM) {
+               if (!current_is_single_threaded())
                        return -EINVAL;
        }
 
@@ -1875,16 +1883,16 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
        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;
-       /*
         * 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)
-- 
2.1.4

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