mnt: Protect the mountpoint hashtable with mount_lock

Protecting the mountpoint hashtable with namespace_sem was sufficient
until a call to umount_mnt was added to mntput_no_expire.  At which
point it became possible for multiple calls of put_mountpoint on
the same hash chain to happen on the same time.

Kristen Johansen <kjlx@templeofstupid.com> reported:
> This can cause a panic when simultaneous callers of put_mountpoint
> attempt to free the same mountpoint.  This occurs because some callers
> hold the mount_hash_lock, while others hold the namespace lock.  Some
> even hold both.
>
> In this submitter's case, the panic manifested itself as a GP fault in
> put_mountpoint() when it called hlist_del() and attempted to dereference
> a m_hash.pprev that had been poisioned by another thread.

Al Viro observed that the simple fix is to switch from using the namespace_sem
to the mount_lock to protect the mountpoint hash table.

I have taken Al's suggested patch moved put_mountpoint in pivot_root
(instead of taking mount_lock an additional time), and have replaced
new_mountpoint with get_mountpoint a function that does the hash table
lookup and addition under the mount_lock.   The introduction of get_mounptoint
ensures that only the mount_lock is needed to manipulate the mountpoint
hashtable.

d_set_mounted is modified to only set DCACHE_MOUNTED if it is not
already set.  This allows get_mountpoint to use the setting of
DCACHE_MOUNTED to ensure adding a struct mountpoint for a dentry
happens exactly once.

Cc: stable@vger.kernel.org
Fixes: ce07d891a089 ("mnt: Honor MNT_LOCKED when detaching mounts")
Reported-by: Krister Johansen <kjlx@templeofstupid.com>
Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

2 files changed, 50 insertions, 21 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 769903dbc19d..95d71eda8142 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1336,8 +1336,11 @@ int d_set_mounted(struct dentry *dentry)
         }
         spin_lock(&dentry->d_lock);
         if (!d_unlinked(dentry)) {
-                dentry->d_flags |= DCACHE_MOUNTED;
-                ret = 0;
+                ret = -EBUSY;
+                if (!d_mountpoint(dentry)) {
+                        dentry->d_flags |= DCACHE_MOUNTED;
+                        ret = 0;
+                }
         }
         spin_unlock(&dentry->d_lock);
 out:
diff --git a/fs/namespace.c b/fs/namespace.c
index b5b1259e064f..487ba30bb5c6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -742,26 +742,50 @@ static struct mountpoint *lookup_mountpoint(struct dentry *dentry)
         return NULL;
 }
 
-static struct mountpoint *new_mountpoint(struct dentry *dentry)
+static struct mountpoint *get_mountpoint(struct dentry *dentry)
 {
-        struct hlist_head *chain = mp_hash(dentry);
-        struct mountpoint *mp;
+        struct mountpoint *mp, *new = NULL;
         int ret;
 
-        mp = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);
-        if (!mp)
+        if (d_mountpoint(dentry)) {
+mountpoint:
+                read_seqlock_excl(&mount_lock);
+                mp = lookup_mountpoint(dentry);
+                read_sequnlock_excl(&mount_lock);
+                if (mp)
+                        goto done;
+        }
+
+        if (!new)
+                new = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);
+        if (!new)
                 return ERR_PTR(-ENOMEM);
 
+
+        /* Exactly one processes may set d_mounted */
         ret = d_set_mounted(dentry);
-        if (ret) {
-                kfree(mp);
-                return ERR_PTR(ret);
-        }
 
-        mp->m_dentry = dentry;
-        mp->m_count = 1;
-        hlist_add_head(&mp->m_hash, chain);
-        INIT_HLIST_HEAD(&mp->m_list);
+        /* Someone else set d_mounted? */
+        if (ret == -EBUSY)
+                goto mountpoint;
+
+        /* The dentry is not available as a mountpoint? */
+        mp = ERR_PTR(ret);
+        if (ret)
+                goto done;
+
+        /* Add the new mountpoint to the hash table */
+        read_seqlock_excl(&mount_lock);
+        new->m_dentry = dentry;
+        new->m_count = 1;
+        hlist_add_head(&new->m_hash, mp_hash(dentry));
+        INIT_HLIST_HEAD(&new->m_list);
+        read_sequnlock_excl(&mount_lock);
+
+        mp = new;
+        new = NULL;
+done:
+        kfree(new);
         return mp;
 }
 
@@ -1595,11 +1619,11 @@ void __detach_mounts(struct dentry *dentry)
         struct mount *mnt;
 
         namespace_lock();
+        lock_mount_hash();
         mp = lookup_mountpoint(dentry);
         if (IS_ERR_OR_NULL(mp))
                 goto out_unlock;
 
-        lock_mount_hash();
         event++;
         while (!hlist_empty(&mp->m_list)) {
                 mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
@@ -1609,9 +1633,9 @@ void __detach_mounts(struct dentry *dentry)
                 }
                 else umount_tree(mnt, UMOUNT_CONNECTED);
         }
-        unlock_mount_hash();
         put_mountpoint(mp);
 out_unlock:
+        unlock_mount_hash();
         namespace_unlock();
 }
 
@@ -2038,9 +2062,7 @@ retry:
         namespace_lock();
         mnt = lookup_mnt(path);
         if (likely(!mnt)) {
-                struct mountpoint *mp = lookup_mountpoint(dentry);
-                if (!mp)
-                        mp = new_mountpoint(dentry);
+                struct mountpoint *mp = get_mountpoint(dentry);
                 if (IS_ERR(mp)) {
                         namespace_unlock();
                         inode_unlock(dentry->d_inode);
@@ -2059,7 +2081,11 @@ retry:
 static void unlock_mount(struct mountpoint *where)
 {
         struct dentry *dentry = where->m_dentry;
+
+        read_seqlock_excl(&mount_lock);
         put_mountpoint(where);
+        read_sequnlock_excl(&mount_lock);
+
         namespace_unlock();
         inode_unlock(dentry->d_inode);
 }
@@ -3135,9 +3161,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
         touch_mnt_namespace(current->nsproxy->mnt_ns);
         /* A moved mount should not expire automatically */
         list_del_init(&new_mnt->mnt_expire);
+        put_mountpoint(root_mp);
         unlock_mount_hash();
         chroot_fs_refs(&root, &new);
-        put_mountpoint(root_mp);
         error = 0;
 out4:
         unlock_mount(old_mp);