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Message-Id: <1445377097-9921-37-git-send-email-kamal@canonical.com>
Date: Tue, 20 Oct 2015 14:36:17 -0700
From: Kamal Mostafa <kamal@...onical.com>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
kernel-team@...ts.ubuntu.com
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.19.y-ckt 036/156] unshare: Unsharing a thread does not require unsharing a vm
3.19.8-ckt8 -stable review patch. If anyone has any objections, please let me know.
------------------
From: "Eric W. Biederman" <ebiederm@...ssion.com>
commit 12c641ab8270f787dfcce08b5f20ce8b65008096 upstream.
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.
Fixes: b2e0d98705e60e45bbb3c0032c48824ad7ae0704 userns: Implement unshare of the user namespace
Reported-by: Ricky Zhou <rickyz@...omium.org>
Reported-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
kernel/fork.c | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 4dc2dda..a4df251 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1803,13 +1803,21 @@ 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(¤t->mm->mm_users) > 1)
+ if (!thread_group_empty(current))
+ return -EINVAL;
+ }
+ if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
+ if (atomic_read(¤t->sighand->count) > 1)
+ return -EINVAL;
+ }
+ if (unshare_flags & CLONE_VM) {
+ if (!current_is_single_threaded())
return -EINVAL;
}
@@ -1878,16 +1886,16 @@ 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)
--
1.9.1
--
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