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Subject[PATCH v2 2/2] cgroup: fix cgroup_path() vs rename() race, take 2
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().

v2: make cgrp->name RCU safe.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
block/blk-cgroup.h | 2 -
include/linux/cgroup.h | 1 +
kernel/cgroup.c | 113 ++++++++++++++++++++++++++++++++++++-------------
3 files changed, 85 insertions(+), 31 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..a3d87d9 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -171,6 +171,7 @@ struct cgroup {

struct cgroup *parent; /* my parent */
struct dentry *dentry; /* cgroup fs entry, RCU protected */
+ char __rcu *name; /* a copy of dentry->d_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 5d4c92e..26c071c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -890,6 +890,7 @@ static void cgroup_free_fn(struct work_struct *work)
simple_xattrs_free(&cgrp->xattrs);

ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
+ kfree(cgrp->name);
kfree(cgrp);
}

@@ -1565,6 +1566,18 @@ static int cgroup_get_rootdir(struct super_block *sb)
return 0;
}

+static char *cgroup_alloc_name(struct dentry *dentry)
+{
+ char *name;
+
+ name = kmalloc(dentry->d_name.len + 1, GFP_KERNEL);
+ if (!name)
+ return NULL;
+ memcpy(name, dentry->d_name.name, dentry->d_name.len);
+ name[dentry->d_name.len] = '\0';
+ return name;
+}
+
static struct dentry *cgroup_mount(struct file_system_type *fs_type,
int flags, const char *unused_dev_name,
void *data)
@@ -1613,13 +1626,19 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
int i;
struct hlist_node *node;
struct css_set *cg;
+ struct dentry *dentry;

BUG_ON(sb->s_root != NULL);

ret = cgroup_get_rootdir(sb);
if (ret)
goto drop_new_super;
- inode = sb->s_root->d_inode;
+ dentry = sb->s_root;
+ inode = dentry->d_inode;
+
+ root_cgrp->name = cgroup_alloc_name(dentry);
+ if (!root_cgrp->name)
+ goto drop_new_super;

mutex_lock(&inode->i_mutex);
mutex_lock(&cgroup_mutex);
@@ -1660,8 +1679,8 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
list_add(&root->root_list, &roots);
root_count++;

- sb->s_root->d_fsdata = root_cgrp;
- root->top_cgroup.dentry = sb->s_root;
+ dentry->d_fsdata = root_cgrp;
+ root->top_cgroup.dentry = dentry;

/* Link the top cgroup in this hierarchy into all
* the css_set objects */
@@ -1751,6 +1770,8 @@ static void cgroup_kill_sb(struct super_block *sb) {
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);

+ synchronize_rcu();
+ kfree(cgrp->name);
simple_xattrs_free(&cgrp->xattrs);

kill_litter_super(sb);
@@ -1771,49 +1792,47 @@ 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 (true) {
+ char *p;
+ int len;
+
+ p = rcu_dereference(cgrp->name);
+ len = strlen(p);
if ((start -= len) < buf)
- return -ENAMETOOLONG;
- memcpy(start, dentry->d_name.name, len);
+ goto fail;
+ memcpy(start, p, len);
+
cgrp = cgrp->parent;
if (!cgrp)
break;

- dentry = cgrp->dentry;
if (!cgrp->parent)
continue;
if (--start < buf)
- return -ENAMETOOLONG;
+ goto fail;
*start = '/';
}
+ ret = 0;
memmove(buf, start, buf + buflen - start);
- return 0;
+fail:
+ rcu_read_unlock();
+ return ret;
}
EXPORT_SYMBOL_GPL(cgroup_path);

@@ -2539,13 +2558,41 @@ 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;
+ char *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);
+
+ synchronize_rcu();
+ kfree(old_name);
+ return 0;
}

static struct simple_xattrs *__d_xattrs(struct dentry *dentry)
@@ -4144,9 +4191,13 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
if (!cgrp)
return -ENOMEM;

+ cgrp->name = cgroup_alloc_name(dentry);
+ if (!cgrp->name)
+ goto err_free_cgrp;
+
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
@@ -4252,6 +4303,8 @@ err_free_all:
deactivate_super(sb);
err_free_id:
ida_simple_remove(&root->cgroup_ida, cgrp->id);
+err_free_name:
+ kfree(cgrp->name);
err_free_cgrp:
kfree(cgrp);
return err;
@@ -4656,6 +4709,8 @@ int __init cgroup_init_early(void)
root_count = 1;
init_task.cgroups = &init_css_set;

+ dummytop->name = "/";
+
init_css_set_link.cg = &init_css_set;
init_css_set_link.cgrp = dummytop;
list_add(&init_css_set_link.cgrp_link_list,
--
1.8.0.2


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