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SubjectRe: [PATCH linux-next] mqueue: fix IPC namespace use-after-free
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Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Dec 19, 2017 at 11:48:19AM +0000, Al Viro wrote:
>> On Tue, Dec 19, 2017 at 11:14:40AM +0100, Giuseppe Scrivano wrote:
>> > mqueue_evict_inode() doesn't access the ipc namespace if it was
>> > already freed. It can happen if in a new IPC namespace the inode was
>> > created without a prior mq_open() which creates the vfsmount used to
>> > access the superblock from mq_clear_sbinfo().
>> >
>> > Keep a direct pointer to the superblock used by the inodes so we can
>> > correctly reset the reference to the IPC namespace being destroyed.
>> >
>> > Bug introduced with 9c583773d03633 ("ipc, mqueue: lazy call
>> > kern_mount_data in new namespaces")
>>
>> And just what will happen in the same scenario if you mount the damn
>> thing in userland without ever calling mq_open(), touch a file there,
>> then unmount and then leave the ipc namespace?
>
> FWIW, the real solution would be to have userland mounts trigger the creation
> of internal one, same as mq_open() would. Something along these lines
> (completely untested, on top of vfs.git#for-next). Care to give it some
> beating?

thanks for the patch. It seems to work after this minor fixup on top of
it:

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 30327e201571..636989a44fae 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -360,7 +360,7 @@ static struct vfsmount *mq_internal_mount(void)
spin_unlock(&mq_lock);
if (!IS_ERR(m))
kern_unmount(m);
- return ns->mq_mnt;
+ return ns->mq_mnt;
}
if (!IS_ERR(m))
ns->mq_mnt = m;
@@ -1560,6 +1560,7 @@ static struct file_system_type mqueue_fs_type = {

int mq_init_ns(struct ipc_namespace *ns)
{
+ ns->mq_mnt = NULL;
ns->mq_queues_count = 0;
ns->mq_queues_max = DFLT_QUEUESMAX;
ns->mq_msg_max = DFLT_MSGMAX;

The only issue I've seen with my version is that if I do:

# unshare -im /bin/sh
# mount -t mqueue mqueue /dev/mqueue
# touch /dev/mqueue/foo
# umount /dev/mqueue
# mount -t mqueue mqueue /dev/mqueue

then /dev/mqueue/foo doesn't exist at this point. Your patch does not
have this problem and /dev/mqueue/foo is again accessible after the
second mount.

Regards,
Giuseppe

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