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eventfs: Delete eventfs_inode when the last dentry is freed 

There exists a race between holding a reference of an eventfs_inode dentry
and the freeing of the eventfs_inode. If user space has a dentry held long
enough, it may still be able to access the dentry's eventfs_inode after it
has been freed.

To prevent this, have he eventfs_inode freed via the last dput() (or via
RCU if the eventfs_inode does not have a dentry).

This means reintroducing the eventfs_inode del_list field at a temporary
place to put the eventfs_inode. It needs to mark it as freed (via the
list) but also must invalidate the dentry immediately as the return from
eventfs_remove_dir() expects that they are. But the dentry invalidation
must not be called under the eventfs_mutex, so it must be done after the
eventfs_inode is marked as free (put on a deletion list).

Link: https://lkml.kernel.org/r/20231101172650.123479767@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ajay Kaher <akaher@vmware.com>
Fixes: 5bdcd5f533 ("eventfs: Implement removal of meta data from eventfs")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (Google) 2023-11-01 13:25:47 -04:00
parent 44365329f8
commit 020010fbfa
2 changed files with 70 additions and 80 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

148
fs/tracefs/event_inode.c
View File

@ -85,8 +85,7 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
mutex_lock(&eventfs_mutex);
ei = dentry->d_fsdata;
/* The LSB is set when the eventfs_inode is being freed */
if (((unsigned long)ei & 1UL) || ei->is_freed) {
if (ei->is_freed) {
/* Do not allow changes if the event is about to be removed. */
mutex_unlock(&eventfs_mutex);
return -ENODEV;
@ -276,35 +275,17 @@ static void free_ei(struct eventfs_inode *ei)
void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
{
struct tracefs_inode *ti_parent;
struct eventfs_inode *ei_child, *tmp;
struct eventfs_inode *ei;
int i;
/* The top level events directory may be freed by this */
if (unlikely(ti->flags & TRACEFS_EVENT_TOP_INODE)) {
LIST_HEAD(ef_del_list);
mutex_lock(&eventfs_mutex);
ei = ti->private;
/* Record all the top level files */
list_for_each_entry_srcu(ei_child, &ei->children, list,
lockdep_is_held(&eventfs_mutex)) {
list_add_tail(&ei_child->del_list, &ef_del_list);
}
/* Nothing should access this, but just in case! */
ti->private = NULL;
mutex_unlock(&eventfs_mutex);
/* Now safely free the top level files and their children */
list_for_each_entry_safe(ei_child, tmp, &ef_del_list, del_list) {
list_del(&ei_child->del_list);
eventfs_remove_dir(ei_child);
}
free_ei(ei);
return;
}
@ -319,14 +300,6 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
if (!ei)
goto out;
/*
* If ei was freed, then the LSB bit is set for d_fsdata.
* But this should not happen, as it should still have a
* ref count that prevents it. Warn in case it does.
*/
if (WARN_ON_ONCE((unsigned long)ei & 1))
goto out;
/* This could belong to one of the files of the ei */
if (ei->dentry != dentry) {
for (i = 0; i < ei->nr_entries; i++) {
@ -336,6 +309,8 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
if (WARN_ON_ONCE(i == ei->nr_entries))
goto out;
ei->d_children[i] = NULL;
} else if (ei->is_freed) {
free_ei(ei);
} else {
ei->dentry = NULL;
}
@ -962,13 +937,65 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
return ERR_PTR(-ENOMEM);
}
static LLIST_HEAD(free_list);
static void eventfs_workfn(struct work_struct *work)
{
struct eventfs_inode *ei, *tmp;
struct llist_node *llnode;
llnode = llist_del_all(&free_list);
llist_for_each_entry_safe(ei, tmp, llnode, llist) {
/* This dput() matches the dget() from unhook_dentry() */
for (int i = 0; i < ei->nr_entries; i++) {
if (ei->d_children[i])
dput(ei->d_children[i]);
}
/* This should only get here if it had a dentry */
if (!WARN_ON_ONCE(!ei->dentry))
dput(ei->dentry);
}
}
static DECLARE_WORK(eventfs_work, eventfs_workfn);
static void free_rcu_ei(struct rcu_head *head)
{
struct eventfs_inode *ei = container_of(head, struct eventfs_inode, rcu);
if (ei->dentry) {
/* Do not free the ei until all references of dentry are gone */
if (llist_add(&ei->llist, &free_list))
queue_work(system_unbound_wq, &eventfs_work);
return;
}
/* If the ei doesn't have a dentry, neither should its children */
for (int i = 0; i < ei->nr_entries; i++) {
WARN_ON_ONCE(ei->d_children[i]);
}
free_ei(ei);
}
static void unhook_dentry(struct dentry *dentry)
{
if (!dentry)
return;
/* Keep the dentry from being freed yet (see eventfs_workfn()) */
dget(dentry);
dentry->d_fsdata = NULL;
d_invalidate(dentry);
mutex_lock(&eventfs_mutex);
/* dentry should now have at least a single reference */
WARN_ONCE((int)d_count(dentry) < 1,
"dentry %px (%s) less than one reference (%d) after invalidate\n",
dentry, dentry->d_name.name, d_count(dentry));
mutex_unlock(&eventfs_mutex);
}
/**
* eventfs_remove_rec - remove eventfs dir or file from list
* @ei: eventfs_inode to be removed.
@ -1006,33 +1033,6 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, struct list_head *head,
list_add_tail(&ei->del_list, head);
}
static void unhook_dentry(struct dentry **dentry, struct dentry **list)
{
if (*dentry) {
unsigned long ptr = (unsigned long)*list;
/* Keep the dentry from being freed yet */
dget(*dentry);
/*
* Paranoid: The dget() above should prevent the dentry
* from being freed and calling eventfs_set_ei_status_free().
* But just in case, set the link list LSB pointer to 1
* and have eventfs_set_ei_status_free() check that to
* make sure that if it does happen, it will not think
* the d_fsdata is an eventfs_inode.
*
* For this to work, no eventfs_inode should be allocated
* on a odd space, as the ef should always be allocated
* to be at least word aligned. Check for that too.
*/
WARN_ON_ONCE(ptr & 1);
(*dentry)->d_fsdata = (void *)(ptr | 1);
*list = *dentry;
*dentry = NULL;
}
}
/**
* eventfs_remove_dir - remove eventfs dir or file from list
* @ei: eventfs_inode to be removed.
@ -1043,39 +1043,27 @@ void eventfs_remove_dir(struct eventfs_inode *ei)
{
struct eventfs_inode *tmp;
LIST_HEAD(ei_del_list);
struct dentry *dentry_list = NULL;
struct dentry *dentry;
int i;
if (!ei)
return;
/*
* Move the deleted eventfs_inodes onto the ei_del_list
* which will also set the is_freed value. Note, this has to be
* done under the eventfs_mutex, but the deletions of
* the dentries must be done outside the eventfs_mutex.
* Hence moving them to this temporary list.
*/
mutex_lock(&eventfs_mutex);
eventfs_remove_rec(ei, &ei_del_list, 0);
list_for_each_entry_safe(ei, tmp, &ei_del_list, del_list) {
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_rcu_ei);
}
mutex_unlock(&eventfs_mutex);
while (dentry_list) {
unsigned long ptr;
dentry = dentry_list;
ptr = (unsigned long)dentry->d_fsdata & ~1UL;
dentry_list = (struct dentry *)ptr;
dentry->d_fsdata = NULL;
d_invalidate(dentry);
mutex_lock(&eventfs_mutex);
/* dentry should now have at least a single reference */
WARN_ONCE((int)d_count(dentry) < 1,
"dentry %px (%s) less than one reference (%d) after invalidate\n",
dentry, dentry->d_name.name, d_count(dentry));
mutex_unlock(&eventfs_mutex);
dput(dentry);
list_for_each_entry_safe(ei, tmp, &ei_del_list, del_list) {
for (int i = 0; i < ei->nr_entries; i++)
unhook_dentry(ei->d_children[i]);
unhook_dentry(ei->dentry);
list_del(&ei->del_list);
call_srcu(&eventfs_srcu, &ei->rcu, free_rcu_ei);
}
}

2
fs/tracefs/internal.h
View File

@ -54,10 +54,12 @@ struct eventfs_inode {
void *data;
/*
* Union - used for deletion
* @llist: for calling dput() if needed after RCU
* @del_list: list of eventfs_inode to delete
* @rcu: eventfs_inode to delete in RCU
*/
union {
struct llist_node llist;
struct list_head del_list;
struct rcu_head rcu;
};