[PATCH v3] eventfs: Test for ei->is_freed when accessing ei->dentry
From: Steven Rostedt
Date: Tue Oct 31 2023 - 11:13:31 EST
From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
The eventfs_inode (ei) is protected by SRCU, but the ei->dentry is not. It
is protected by the eventfs_mutex. Anytime the eventfs_mutex is released,
and access to the ei->dentry needs to be done, it should first check if
ei->is_freed is set under the eventfs_mutex. If it is, then the ei->dentry
is invalid and must not be used. The ei->dentry must only be accessed
under the eventfs_mutex and after checking if ei->is_freed is set.
When the ei is being freed, it will (under the eventfs_mutex) set is_freed
and at the same time move the dentry to a free list to be cleared after
the eventfs_mutex is released. This means that any access to the
ei->dentry must check first if ei->is_freed is set, because if it is, then
the dentry is on its way to be freed.
Also add comments to describe this better.
Link: https://lore.kernel.org/all/CA+G9fYt6pY+tMZEOg=SoEywQOe19fGP3uR15SGowkdK+_X85Cg@xxxxxxxxxxxxxx/
Link: https://lore.kernel.org/all/CA+G9fYuDP3hVQ3t7FfrBAjd_WFVSurMgCepTxunSJf=MTe=6aA@xxxxxxxxxxxxxx/
Fixes: 5790b1fb3d672 ("eventfs: Remove eventfs_file and just use eventfs_inode")
Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Reported-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v2: https://lore.kernel.org/linux-trace-kernel/20231028164650.4f5ea18a@xxxxxxxxxxxxxxxxxxxx
- Since I added free_ei() separately, might as well use it for where it frees the
top eventfs_inode too.
fs/tracefs/event_inode.c | 69 ++++++++++++++++++++++++++++++++--------
fs/tracefs/internal.h | 3 +-
2 files changed, 57 insertions(+), 15 deletions(-)
diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 4d2da7480e5f..a62cb2298350 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -24,7 +24,20 @@
#include <linux/delay.h>
#include "internal.h"
+/*
+ * eventfs_mutex protects the eventfs_inode (ei) dentry. Any access
+ * to the ei->dentry must be done under this mutex and after checking
+ * if ei->is_freed is not set. The ei->dentry is released under the
+ * mutex at the same time ei->is_freed is set. If ei->is_freed is set
+ * then the ei->dentry is invalid.
+ */
static DEFINE_MUTEX(eventfs_mutex);
+
+/*
+ * The eventfs_inode (ei) itself is protected by SRCU. It is released from
+ * its parent's list and will have is_freed set (under eventfs_mutex).
+ * After the SRCU grace period is over, the ei may be freed.
+ */
DEFINE_STATIC_SRCU(eventfs_srcu);
static struct dentry *eventfs_root_lookup(struct inode *dir,
@@ -129,6 +142,13 @@ static struct dentry *create_dir(const char *name, struct dentry *parent)
return eventfs_end_creating(dentry);
}
+static void free_ei(struct eventfs_inode *ei)
+{
+ kfree_const(ei->name);
+ kfree(ei->d_children);
+ kfree(ei);
+}
+
/**
* eventfs_set_ei_status_free - remove the dentry reference from an eventfs_inode
* @ti: the tracefs_inode of the dentry
@@ -168,9 +188,7 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
eventfs_remove_dir(ei_child);
}
- kfree_const(ei->name);
- kfree(ei->d_children);
- kfree(ei);
+ free_ei(ei);
return;
}
@@ -234,6 +252,10 @@ create_file_dentry(struct eventfs_inode *ei, struct dentry **e_dentry,
bool invalidate = false;
mutex_lock(&eventfs_mutex);
+ if (ei->is_freed) {
+ mutex_unlock(&eventfs_mutex);
+ return NULL;
+ }
/* If the e_dentry already has a dentry, use it */
if (*e_dentry) {
/* lookup does not need to up the ref count */
@@ -307,6 +329,8 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
struct eventfs_inode *ei_child;
struct tracefs_inode *ti;
+ lockdep_assert_held(&eventfs_mutex);
+
/* srcu lock already held */
/* fill parent-child relation */
list_for_each_entry_srcu(ei_child, &ei->children, list,
@@ -320,6 +344,7 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
/**
* create_dir_dentry - Create a directory dentry for the eventfs_inode
+ * @pei: The eventfs_inode parent of ei.
* @ei: The eventfs_inode to create the directory for
* @parent: The dentry of the parent of this directory
* @lookup: True if this is called by the lookup code
@@ -327,12 +352,17 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
* This creates and attaches a directory dentry to the eventfs_inode @ei.
*/
static struct dentry *
-create_dir_dentry(struct eventfs_inode *ei, struct dentry *parent, bool lookup)
+create_dir_dentry(struct eventfs_inode *pei, struct eventfs_inode *ei,
+ struct dentry *parent, bool lookup)
{
bool invalidate = false;
struct dentry *dentry = NULL;
mutex_lock(&eventfs_mutex);
+ if (pei->is_freed || ei->is_freed) {
+ mutex_unlock(&eventfs_mutex);
+ return NULL;
+ }
if (ei->dentry) {
/* If the dentry already has a dentry, use it */
dentry = ei->dentry;
@@ -435,7 +465,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
*/
mutex_lock(&eventfs_mutex);
ei = READ_ONCE(ti->private);
- if (ei)
+ if (ei && !ei->is_freed)
ei_dentry = READ_ONCE(ei->dentry);
mutex_unlock(&eventfs_mutex);
@@ -449,7 +479,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
if (strcmp(ei_child->name, name) != 0)
continue;
ret = simple_lookup(dir, dentry, flags);
- create_dir_dentry(ei_child, ei_dentry, true);
+ create_dir_dentry(ei, ei_child, ei_dentry, true);
created = true;
break;
}
@@ -583,7 +613,7 @@ static int dcache_dir_open_wrapper(struct inode *inode, struct file *file)
list_for_each_entry_srcu(ei_child, &ei->children, list,
srcu_read_lock_held(&eventfs_srcu)) {
- d = create_dir_dentry(ei_child, parent, false);
+ d = create_dir_dentry(ei, ei_child, parent, false);
if (d) {
ret = add_dentries(&dentries, d, cnt);
if (ret < 0)
@@ -700,12 +730,20 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
ei->nr_entries = size;
ei->data = data;
INIT_LIST_HEAD(&ei->children);
+ INIT_LIST_HEAD(&ei->list);
mutex_lock(&eventfs_mutex);
- list_add_tail(&ei->list, &parent->children);
- ei->d_parent = parent->dentry;
+ if (!parent->is_freed) {
+ list_add_tail(&ei->list, &parent->children);
+ ei->d_parent = parent->dentry;
+ }
mutex_unlock(&eventfs_mutex);
+ /* Was the parent freed? */
+ if (list_empty(&ei->list)) {
+ free_ei(ei);
+ ei = NULL;
+ }
return ei;
}
@@ -787,13 +825,11 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
return ERR_PTR(-ENOMEM);
}
-static void free_ei(struct rcu_head *head)
+static void free_rcu_ei(struct rcu_head *head)
{
struct eventfs_inode *ei = container_of(head, struct eventfs_inode, rcu);
- kfree_const(ei->name);
- kfree(ei->d_children);
- kfree(ei);
+ free_ei(ei);
}
/**
@@ -880,7 +916,12 @@ void eventfs_remove_dir(struct eventfs_inode *ei)
for (i = 0; i < ei->nr_entries; i++)
unhook_dentry(&ei->d_children[i], &dentry_list);
unhook_dentry(&ei->dentry, &dentry_list);
- call_srcu(&eventfs_srcu, &ei->rcu, free_ei);
+ /*
+ * Note, ei->is_freed is a union along with ei->rcu
+ * and ei->del_list. When the ei is added to either
+ * of those lists, it automatically sets ei->is_freed.
+ */
+ call_srcu(&eventfs_srcu, &ei->rcu, free_rcu_ei);
}
mutex_unlock(&eventfs_mutex);
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 64fde9490f52..21a1fa682b74 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -30,7 +30,7 @@ struct eventfs_inode {
const struct eventfs_entry *entries;
const char *name;
struct list_head children;
- struct dentry *dentry;
+ struct dentry *dentry; /* Check is_freed to access */
struct dentry *d_parent;
struct dentry **d_children;
void *data;
@@ -39,6 +39,7 @@ struct eventfs_inode {
* @del_list: list of eventfs_inode to delete
* @rcu: eventfs_inode to delete in RCU
* @is_freed: node is freed if one of the above is set
+ * Note if is_freed is set, then dentry is corrupted.
*/
union {
struct list_head del_list;
--
2.42.0