From 12c641ab8270f787dfcce08b5f20ce8b65008096 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Mon, 10 Aug 2015 17:35:07 -0500 Subject: [PATCH] unshare: Unsharing a thread does not require unsharing a vm 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 Reported-by: Kees Cook Reviewed-by: Kees Cook Signed-off-by: "Eric W. Biederman" --- kernel/fork.c | 28 ++++++++++++++++++---------- 1 file changed, 18 insertions(+), 10 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 1bfefc6f96a4..d544ae97f999 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1866,13 +1866,21 @@ 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; } @@ -1940,16 +1948,16 @@ 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. */ -- 2.39.5