[PATCH v5 3/7] eventfs: Test for ei->is_freed when accessing ei->dentry

Steven Rostedt
2023-10-31T22:34:32Z

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@mail.gmail.com/
Link: https://lore.kernel.org/all/CA+G9fYuDP3hVQ3t7FfrBAjd_WFVSurMgCepTxunSJf=MTe=6aA@mail.gmail.com/

Fixes: 5790b1fb3d672 ("eventfs: Remove eventfs_file and just use eventfs_inode")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Beau Belgrave <beaub@linux.microsoft.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v4: https://lkml.kernel.org/r/20231031193428.347017769@goodmis.org

- Rebased to this series

fs/tracefs/event_inode.c | 45 ++++++++++++++++++++++++++++++++++------
fs/tracefs/internal.h | 3 ++-
2 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 0331d9bd568b..b28f240bbb6d 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,
@@ -239,6 +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 */
@@ -312,6 +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,
@@ -325,6 +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
@@ -332,12 +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;
@@ -440,7 +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);

@@ -454,7 +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;
}
@@ -588,7 +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)
@@ -705,12 +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;
}

diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index c7d88aaa949f..5a98e87dd3d1 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -24,6 +24,7 @@ struct tracefs_inode {
* @d_children: The array of dentries to represent the files when created
* @data: The private data to pass to the callbacks
* @is_freed: Flag set if the eventfs is on its way to be freed
+ * Note if is_freed is set, then dentry is corrupted.
* @nr_entries: The number of items in @entries
*/
struct eventfs_inode {
@@ -31,7 +32,7 @@ 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;
--
2.42.0