cgroup: fix cgroup_path() vs rename() race

rename() will change dentry->d_name. The result of this race can
be worse than seeing partially rewritten name, but we might access
a stale pointer because rename() will re-allocate memory to hold
a longer name.

As accessing dentry->name must be protected by dentry->d_lock or
parent inode's i_mutex, while on the other hand cgroup-path() can
be called with some irq-safe spinlocks held, we can't generate
cgroup path using dentry->d_name.

Alternatively we make a copy of dentry->d_name and save it in
cgrp->name when a cgroup is created, and update cgrp->name at
rename().

v5: use flexible array instead of zero-size array.
v4: - allocate root_cgroup_name and all root_cgroup->name points to it.
    - add cgroup_name() wrapper.
v3: use kfree_rcu() instead of synchronize_rcu() in user-visible path.
v2: make cgrp->name RCU safe.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 76 insertions, 30 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a32f9432666c..50682168abc2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -238,6 +238,8 @@ static DEFINE_SPINLOCK(hierarchy_id_lock);
 /* dummytop is a shorthand for the dummy hierarchy's top cgroup */
 #define dummytop (&rootnode.top_cgroup)
 
+static struct cgroup_name root_cgroup_name = { .name = "/" };
+
 /* This flag indicates whether tasks in the fork and exit paths should
  * check for fork/exit handlers to call. This avoids us having to do
  * extra work in the fork/exit path if none of the subsystems need to
@@ -859,6 +861,17 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
         return inode;
 }
 
+static struct cgroup_name *cgroup_alloc_name(struct dentry *dentry)
+{
+        struct cgroup_name *name;
+
+        name = kmalloc(sizeof(*name) + dentry->d_name.len + 1, GFP_KERNEL);
+        if (!name)
+                return NULL;
+        strcpy(name->name, dentry->d_name.name);
+        return name;
+}
+
 static void cgroup_free_fn(struct work_struct *work)
 {
         struct cgroup *cgrp = container_of(work, struct cgroup, free_work);
@@ -889,6 +902,7 @@ static void cgroup_free_fn(struct work_struct *work)
         simple_xattrs_free(&cgrp->xattrs);
 
         ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
+        kfree(rcu_dereference_raw(cgrp->name));
         kfree(cgrp);
 }
 
@@ -1421,6 +1435,7 @@ static void init_cgroup_root(struct cgroupfs_root *root)
         INIT_LIST_HEAD(&root->allcg_list);
         root->number_of_cgroups = 1;
         cgrp->root = root;
+        cgrp->name = &root_cgroup_name;
         cgrp->top_cgroup = cgrp;
         init_cgroup_housekeeping(cgrp);
         list_add_tail(&cgrp->allcg_node, &root->allcg_list);
@@ -1769,49 +1784,45 @@ static struct kobject *cgroup_kobj;
  * @buf: the buffer to write the path into
  * @buflen: the length of the buffer
  *
- * Called with cgroup_mutex held or else with an RCU-protected cgroup
- * reference.  Writes path of cgroup into buf.  Returns 0 on success,
- * -errno on error.
+ * Writes path of cgroup into buf.  Returns 0 on success, -errno on error.
+ *
+ * We can't generate cgroup path using dentry->d_name, as accessing
+ * dentry->name must be protected by irq-unsafe dentry->d_lock or parent
+ * inode's i_mutex, while on the other hand cgroup_path() can be called
+ * with some irq-safe spinlocks held.
  */
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 {
-        struct dentry *dentry = cgrp->dentry;
+        int ret = -ENAMETOOLONG;
         char *start;
 
-        rcu_lockdep_assert(rcu_read_lock_held() || cgroup_lock_is_held(),
-                           "cgroup_path() called without proper locking");
-
-        if (cgrp == dummytop) {
-                /*
-                 * Inactive subsystems have no dentry for their root
-                 * cgroup
-                 */
-                strcpy(buf, "/");
-                return 0;
-        }
-
         start = buf + buflen - 1;
-
         *start = '\0';
-        for (;;) {
-                int len = dentry->d_name.len;
 
+        rcu_read_lock();
+        while (cgrp) {
+                const char *name = cgroup_name(cgrp);
+                int len;
+
+                len = strlen(name);
                 if ((start -= len) < buf)
-                        return -ENAMETOOLONG;
-                memcpy(start, dentry->d_name.name, len);
-                cgrp = cgrp->parent;
-                if (!cgrp)
-                        break;
+                        goto out;
+                memcpy(start, name, len);
 
-                dentry = cgrp->dentry;
                 if (!cgrp->parent)
-                        continue;
+                        break;
+
                 if (--start < buf)
-                        return -ENAMETOOLONG;
+                        goto out;
                 *start = '/';
+
+                cgrp = cgrp->parent;
         }
+        ret = 0;
         memmove(buf, start, buf + buflen - start);
-        return 0;
+out:
+        rcu_read_unlock();
+        return ret;
 }
 EXPORT_SYMBOL_GPL(cgroup_path);
 
@@ -2537,13 +2548,40 @@ static int cgroup_file_release(struct inode *inode, struct file *file)
 static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
                             struct inode *new_dir, struct dentry *new_dentry)
 {
+        int ret;
+        struct cgroup_name *name, *old_name;
+        struct cgroup *cgrp;
+
+        /*
+         * It's convinient to use parent dir's i_mutex to protected
+         * cgrp->name.
+         */
+        lockdep_assert_held(&old_dir->i_mutex);
+
         if (!S_ISDIR(old_dentry->d_inode->i_mode))
                 return -ENOTDIR;
         if (new_dentry->d_inode)
                 return -EEXIST;
         if (old_dir != new_dir)
                 return -EIO;
-        return simple_rename(old_dir, old_dentry, new_dir, new_dentry);
+
+        cgrp = __d_cgrp(old_dentry);
+
+        name = cgroup_alloc_name(new_dentry);
+        if (!name)
+                return -ENOMEM;
+
+        ret = simple_rename(old_dir, old_dentry, new_dir, new_dentry);
+        if (ret) {
+                kfree(name);
+                return ret;
+        }
+
+        old_name = cgrp->name;
+        rcu_assign_pointer(cgrp->name, name);
+
+        kfree_rcu(old_name, rcu_head);
+        return 0;
 }
 
 static struct simple_xattrs *__d_xattrs(struct dentry *dentry)
@@ -4158,6 +4196,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                              umode_t mode)
 {
         struct cgroup *cgrp;
+        struct cgroup_name *name;
         struct cgroupfs_root *root = parent->root;
         int err = 0;
         struct cgroup_subsys *ss;
@@ -4168,9 +4207,14 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
         if (!cgrp)
                 return -ENOMEM;
 
+        name = cgroup_alloc_name(dentry);
+        if (!name)
+                goto err_free_cgrp;
+        rcu_assign_pointer(cgrp->name, name);
+
         cgrp->id = ida_simple_get(&root->cgroup_ida, 1, 0, GFP_KERNEL);
         if (cgrp->id < 0)
-                goto err_free_cgrp;
+                goto err_free_name;
 
         /*
          * Only live parents can have children.  Note that the liveliness
@@ -4276,6 +4320,8 @@ err_free_all:
         deactivate_super(sb);
 err_free_id:
         ida_simple_remove(&root->cgroup_ida, cgrp->id);
+err_free_name:
+        kfree(rcu_dereference_raw(cgrp->name));
 err_free_cgrp:
         kfree(cgrp);
         return err;