Commit 65dff759 authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

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: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 6dbe51c2
......@@ -247,9 +247,7 @@ 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;
......
......@@ -150,6 +150,11 @@ enum {
CGRP_CPUSET_CLONE_CHILDREN,
};
struct cgroup_name {
struct rcu_head rcu_head;
char name[];
};
struct cgroup {
unsigned long flags; /* "unsigned long" so bitops work */
......@@ -172,6 +177,19 @@ 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.
*
* Normally you should use cgroup_name() wrapper rather than
* access it directly.
*/
struct cgroup_name __rcu *name;
/* Private pointers for each registered subsystem */
struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
......@@ -404,6 +422,12 @@ struct cgroup_scanner {
void *data;
};
/* Caller should hold rcu_read_lock() */
static inline const char *cgroup_name(const struct cgroup *cgrp)
{
return rcu_dereference(cgrp->name)->name;
}
int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
......
......@@ -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;
......
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