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author | Eric W. Biederman <ebiederm@xmission.com> | 2015-08-10 17:35:07 -0500 |
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committer | Luis Henriques <luis.henriques@canonical.com> | 2015-09-28 10:21:39 +0100 |
commit | 6d43a10ee757c4c3ed3df89dbc55f0c7b1be3834 (patch) | |
tree | 582344850d7b95f6695d448ec128a314be60f729 | |
parent | 93feb4de6765a78f9596aba5e2f4cc8deb01f09a (diff) | |
download | linux-stable-6d43a10ee757c4c3ed3df89dbc55f0c7b1be3834.tar.gz linux-stable-6d43a10ee757c4c3ed3df89dbc55f0c7b1be3834.tar.bz2 linux-stable-6d43a10ee757c4c3ed3df89dbc55f0c7b1be3834.zip |
unshare: Unsharing a thread does not require unsharing a vm
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@chromium.org>
Reported-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
-rw-r--r-- | kernel/fork.c | 28 |
1 files changed, 18 insertions, 10 deletions
diff --git a/kernel/fork.c b/kernel/fork.c index b41958b0cb67..a4ca06e80972 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1753,13 +1753,21 @@ 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; } @@ -1828,16 +1836,16 @@ 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) |