[PATCH v3 1/3] cgroup: fix cgroup_path() vs rename() race

Li Zefan
2013-02-25T06:18:22Z

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().

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>
---
block/blk-cgroup.h | 2 -
include/linux/cgroup.h | 17 +++++++++
kernel/cgroup.c | 101 ++++++++++++++++++++++++++++++++++++-------------
3 files changed, 91 insertions(+), 29 deletions(-)

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 2459730..e2e3404 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -216,9 +216,7 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
{
int ret;

- rcu_read_lock();
ret = cgroup_path(blkg->blkcg->css.cgroup, buf, buflen);
- rcu_read_unlock();
if (ret)
strncpy(buf, "<unavailable>", buflen);
return ret;
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 900af59..7f21bad 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -150,6 +150,11 @@ enum {
CGRP_CPUSET_CLONE_CHILDREN,
};

+struct cgroup_name {
+ struct rcu_head rcu_head;
+ char name[0];
+};
+
struct cgroup {
unsigned long flags; /* "unsigned long" so bitops work */

@@ -172,6 +177,18 @@ struct cgroup {
struct cgroup *parent; /* my parent */
struct dentry *dentry; /* cgroup fs entry, RCU protected */

+ /*
+ * This is a copy of dentry->d_name, and it's needed because
+ * we can't use dentry->d_name in cgroup_path().
+ *
+ * You must acquire rcu_read_lock() to access cgrp->name, and
+ * the only place that can change it is rename(), which is
+ * protected by parent dir's i_mutex.
+ *
+ * The name of the root cgroup is "/", and @name == NULL.
+ */
+ struct cgroup_name __rcu *name;
+
/* Private pointers for each registered subsystem */
struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b5c6432..eb3c280 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -860,6 +860,17 @@ 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);
@@ -890,6 +901,7 @@ 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);
}

@@ -1771,49 +1783,49 @@ 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
- */
+ if (!cgrp->parent) {
strcpy(buf, "/");
return 0;
}

start = buf + buflen - 1;
-
*start = '\0';
- for (;;) {
- int len = dentry->d_name.len;

+ rcu_read_lock();
+ while (cgrp->parent) {
+ struct cgroup_name *cname = rcu_dereference(cgrp->name);
+ char *name;
+ int len;
+
+ name = cname->name;
+ 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;
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);

@@ -2539,13 +2551,40 @@ 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)
@@ -4160,6 +4199,7 @@ 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;
@@ -4170,9 +4210,14 @@ 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
@@ -4278,6 +4323,8 @@ 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;
--
1.8.0.2