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SubjectRe: [PATCH linux-next] mqueue: fix IPC namespace use-after-free
On Tue, Dec 19, 2017 at 07:40:43PM +0100, Giuseppe Scrivano wrote:
> Giuseppe Scrivano <gscrivan@redhat.com> writes:
>
> > 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.
>
> although, how much is that of an issue? Is there any other way to delay

You do realize that with your patch you would end up with worse than that -
mount/touch/umount and now you've got ->mq_sb non-NULL and pointing to freed
super_block. With really unpleasant effects when you quit that ipcns...

> the cost of kern_mount_data()? Most containers have /dev/mqueue mounted
> but it is not really going to be used.

_What_ cost? At mount(2) time you are setting the superblock up anyway, so
what would you be delaying? kmem_cache_alloc() for struct mount and assignments
to its fields? That's noise; if anything, I would expect the main cost with
a plenty of containers to be in sget() scanning the list of mqueue superblocks.
And we can get rid of that, while we are at it - to hell with mount_ns(), with
that approach we can just use mount_nodev() instead. The logics in
mq_internal_mount() will deal with multiple instances - if somebody has already
triggered creation of internal mount, all subsequent calls in that ipcns will
end up avoiding kern_mount_data() entirely. And if you have two callers
racing - sure, you will get two superblocks. Not for long, though - the first
one to get to setting ->mq_mnt (serialized on mq_lock) wins, the second loses
and prompty destroys his vfsmount and superblock. I seriously suspect that
variant below would cut down on the cost a whole lot more - as it is, we have
the total of O(N^2) spent in the loop inside of sget_userns() when we create
N ipcns and mount in each of those; this patch should cut that to O(N)...

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 10b82338415b..e9870b0cda29 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -325,8 +325,9 @@ static struct inode *mqueue_get_inode(struct super_block *sb,
static int mqueue_fill_super(struct super_block *sb, void *data, int silent)
{
struct inode *inode;
- struct ipc_namespace *ns = sb->s_fs_info;
+ struct ipc_namespace *ns = data;

+ sb->s_fs_info = ns;
sb->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
sb->s_blocksize = PAGE_SIZE;
sb->s_blocksize_bits = PAGE_SHIFT;
@@ -343,18 +344,44 @@ static int mqueue_fill_super(struct super_block *sb, void *data, int silent)
return 0;
}

+static struct file_system_type mqueue_fs_type;
+/*
+ * Return value is pinned only by reference in ->mq_mnt; it will
+ * live until ipcns dies. Caller does not need to drop it.
+ */
+static struct vfsmount *mq_internal_mount(void)
+{
+ struct ipc_namespace *ns = current->nsproxy->ipc_ns;
+ struct vfsmount *m = ns->mq_mnt;
+ if (m)
+ return m;
+ m = kern_mount_data(&mqueue_fs_type, ns);
+ spin_lock(&mq_lock);
+ if (unlikely(ns->mq_mnt)) {
+ spin_unlock(&mq_lock);
+ if (!IS_ERR(m))
+ kern_unmount(m);
+ return ns->mq_mnt;
+ }
+ if (!IS_ERR(m))
+ ns->mq_mnt = m;
+ spin_unlock(&mq_lock);
+ return m;
+}
+
static struct dentry *mqueue_mount(struct file_system_type *fs_type,
int flags, const char *dev_name,
void *data)
{
- struct ipc_namespace *ns;
- if (flags & MS_KERNMOUNT) {
- ns = data;
- data = NULL;
- } else {
- ns = current->nsproxy->ipc_ns;
- }
- return mount_ns(fs_type, flags, data, ns, ns->user_ns, mqueue_fill_super);
+ struct vfsmount *m;
+ if (flags & MS_KERNMOUNT)
+ return mount_nodev(fs_type, flags, data, mqueue_fill_super);
+ m = mq_internal_mount();
+ if (IS_ERR(m))
+ return ERR_CAST(m);
+ atomic_inc(&m->mnt_sb->s_active);
+ down_write(&m->mnt_sb->s_umount);
+ return dget(m->mnt_root);
}

static void init_once(void *foo)
@@ -743,13 +770,16 @@ static int prepare_open(struct dentry *dentry, int oflag, int ro,
static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
struct mq_attr *attr)
{
- struct vfsmount *mnt = current->nsproxy->ipc_ns->mq_mnt;
- struct dentry *root = mnt->mnt_root;
+ struct vfsmount *mnt = mq_internal_mount();
+ struct dentry *root;
struct filename *name;
struct path path;
int fd, error;
int ro;

+ if (IS_ERR(mnt))
+ return PTR_ERR(mnt);
+
audit_mq_open(oflag, mode, attr);

if (IS_ERR(name = getname(u_name)))
@@ -760,6 +790,7 @@ static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
goto out_putname;

ro = mnt_want_write(mnt); /* we'll drop it in any case */
+ root = mnt->mnt_root;
inode_lock(d_inode(root));
path.dentry = lookup_one_len(name->name, root, strlen(name->name));
if (IS_ERR(path.dentry)) {
@@ -1534,28 +1565,26 @@ int mq_init_ns(struct ipc_namespace *ns)
ns->mq_msgsize_max = DFLT_MSGSIZEMAX;
ns->mq_msg_default = DFLT_MSG;
ns->mq_msgsize_default = DFLT_MSGSIZE;
+ ns->mq_mnt = NULL;

- ns->mq_mnt = kern_mount_data(&mqueue_fs_type, ns);
- if (IS_ERR(ns->mq_mnt)) {
- int err = PTR_ERR(ns->mq_mnt);
- ns->mq_mnt = NULL;
- return err;
- }
return 0;
}

void mq_clear_sbinfo(struct ipc_namespace *ns)
{
- ns->mq_mnt->mnt_sb->s_fs_info = NULL;
+ if (ns->mq_mnt)
+ ns->mq_mnt->mnt_sb->s_fs_info = NULL;
}

void mq_put_mnt(struct ipc_namespace *ns)
{
- kern_unmount(ns->mq_mnt);
+ if (ns->mq_mnt)
+ kern_unmount(ns->mq_mnt);
}

static int __init init_mqueue_fs(void)
{
+ struct vfsmount *m;
int error;

mqueue_inode_cachep = kmem_cache_create("mqueue_inode_cache",
@@ -1577,6 +1606,10 @@ static int __init init_mqueue_fs(void)
if (error)
goto out_filesystem;

+ m = kern_mount_data(&mqueue_fs_type, &init_ipc_ns);
+ if (IS_ERR(m))
+ goto out_filesystem;
+ init_ipc_ns.mq_mnt = m;
return 0;

out_filesystem:
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