linux/fs/notify/fanotify/fanotify.c
Lino Sanfilippo 09e5f14e57 fanotify: on group destroy allow all waiters to bypass permission check
When fanotify_release() is called, there may still be processes waiting for
access permission. Currently only processes for which an event has already been
queued into the groups access list will be woken up.  Processes for which no
event has been queued will continue to sleep and thus cause a deadlock when
fsnotify_put_group() is called.
Furthermore there is a race allowing further processes to be waiting on the
access wait queue after wake_up (if they arrive before clear_marks_by_group()
is called).
This patch corrects this by setting a flag to inform processes that the group
is about to be destroyed and thus not to wait for access permission.

[additional changelog from eparis]
Lets think about the 4 relevant code paths from the PoV of the
'operator' 'listener' 'responder' and 'closer'.  Where operator is the
process doing an action (like open/read) which could require permission.
Listener is the task (or in this case thread) slated with reading from
the fanotify file descriptor.  The 'responder' is the thread responsible
for responding to access requests.  'Closer' is the thread attempting to
close the fanotify file descriptor.

The 'operator' is going to end up in:
fanotify_handle_event()
  get_response_from_access()
    (THIS BLOCKS WAITING ON USERSPACE)

The 'listener' interesting code path
fanotify_read()
  copy_event_to_user()
    prepare_for_access_response()
      (THIS CREATES AN fanotify_response_event)

The 'responder' code path:
fanotify_write()
  process_access_response()
    (REMOVE A fanotify_response_event, SET RESPONSE, WAKE UP 'operator')

The 'closer':
fanotify_release()
  (SUPPOSED TO CLEAN UP THE REST OF THIS MESS)

What we have today is that in the closer we remove all of the
fanotify_response_events and set a bit so no more response events are
ever created in prepare_for_access_response().

The bug is that we never wake all of the operators up and tell them to
move along.  You fix that in fanotify_get_response_from_access().  You
also fix other operators which haven't gotten there yet.  So I agree
that's a good fix.
[/additional changelog from eparis]

[remove additional changes to minimize patch size]
[move initialization so it was inside CONFIG_FANOTIFY_PERMISSION]

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Eric Paris <eparis@redhat.com>
2010-12-07 16:14:22 -05:00

228 lines
6.1 KiB
C

#include <linux/fanotify.h>
#include <linux/fdtable.h>
#include <linux/fsnotify_backend.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h> /* UINT_MAX */
#include <linux/mount.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/wait.h>
static bool should_merge(struct fsnotify_event *old, struct fsnotify_event *new)
{
pr_debug("%s: old=%p new=%p\n", __func__, old, new);
if (old->to_tell == new->to_tell &&
old->data_type == new->data_type &&
old->tgid == new->tgid) {
switch (old->data_type) {
case (FSNOTIFY_EVENT_PATH):
if ((old->path.mnt == new->path.mnt) &&
(old->path.dentry == new->path.dentry))
return true;
case (FSNOTIFY_EVENT_NONE):
return true;
default:
BUG();
};
}
return false;
}
/* and the list better be locked by something too! */
static struct fsnotify_event *fanotify_merge(struct list_head *list,
struct fsnotify_event *event)
{
struct fsnotify_event_holder *test_holder;
struct fsnotify_event *test_event = NULL;
struct fsnotify_event *new_event;
pr_debug("%s: list=%p event=%p\n", __func__, list, event);
list_for_each_entry_reverse(test_holder, list, event_list) {
if (should_merge(test_holder->event, event)) {
test_event = test_holder->event;
break;
}
}
if (!test_event)
return NULL;
fsnotify_get_event(test_event);
/* if they are exactly the same we are done */
if (test_event->mask == event->mask)
return test_event;
/*
* if the refcnt == 2 this is the only queue
* for this event and so we can update the mask
* in place.
*/
if (atomic_read(&test_event->refcnt) == 2) {
test_event->mask |= event->mask;
return test_event;
}
new_event = fsnotify_clone_event(test_event);
/* done with test_event */
fsnotify_put_event(test_event);
/* couldn't allocate memory, merge was not possible */
if (unlikely(!new_event))
return ERR_PTR(-ENOMEM);
/* build new event and replace it on the list */
new_event->mask = (test_event->mask | event->mask);
fsnotify_replace_event(test_holder, new_event);
/* we hold a reference on new_event from clone_event */
return new_event;
}
#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
static int fanotify_get_response_from_access(struct fsnotify_group *group,
struct fsnotify_event *event)
{
int ret;
pr_debug("%s: group=%p event=%p\n", __func__, group, event);
wait_event(group->fanotify_data.access_waitq, event->response ||
atomic_read(&group->fanotify_data.bypass_perm));
if (!event->response) /* bypass_perm set */
return 0;
/* userspace responded, convert to something usable */
spin_lock(&event->lock);
switch (event->response) {
case FAN_ALLOW:
ret = 0;
break;
case FAN_DENY:
default:
ret = -EPERM;
}
event->response = 0;
spin_unlock(&event->lock);
pr_debug("%s: group=%p event=%p about to return ret=%d\n", __func__,
group, event, ret);
return ret;
}
#endif
static int fanotify_handle_event(struct fsnotify_group *group,
struct fsnotify_mark *inode_mark,
struct fsnotify_mark *fanotify_mark,
struct fsnotify_event *event)
{
int ret = 0;
struct fsnotify_event *notify_event = NULL;
BUILD_BUG_ON(FAN_ACCESS != FS_ACCESS);
BUILD_BUG_ON(FAN_MODIFY != FS_MODIFY);
BUILD_BUG_ON(FAN_CLOSE_NOWRITE != FS_CLOSE_NOWRITE);
BUILD_BUG_ON(FAN_CLOSE_WRITE != FS_CLOSE_WRITE);
BUILD_BUG_ON(FAN_OPEN != FS_OPEN);
BUILD_BUG_ON(FAN_EVENT_ON_CHILD != FS_EVENT_ON_CHILD);
BUILD_BUG_ON(FAN_Q_OVERFLOW != FS_Q_OVERFLOW);
BUILD_BUG_ON(FAN_OPEN_PERM != FS_OPEN_PERM);
BUILD_BUG_ON(FAN_ACCESS_PERM != FS_ACCESS_PERM);
BUILD_BUG_ON(FAN_ONDIR != FS_ISDIR);
pr_debug("%s: group=%p event=%p\n", __func__, group, event);
notify_event = fsnotify_add_notify_event(group, event, NULL, fanotify_merge);
if (IS_ERR(notify_event))
return PTR_ERR(notify_event);
#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
if (event->mask & FAN_ALL_PERM_EVENTS) {
/* if we merged we need to wait on the new event */
if (notify_event)
event = notify_event;
ret = fanotify_get_response_from_access(group, event);
}
#endif
if (notify_event)
fsnotify_put_event(notify_event);
return ret;
}
static bool fanotify_should_send_event(struct fsnotify_group *group,
struct inode *to_tell,
struct fsnotify_mark *inode_mark,
struct fsnotify_mark *vfsmnt_mark,
__u32 event_mask, void *data, int data_type)
{
__u32 marks_mask, marks_ignored_mask;
struct path *path = data;
pr_debug("%s: group=%p to_tell=%p inode_mark=%p vfsmnt_mark=%p "
"mask=%x data=%p data_type=%d\n", __func__, group, to_tell,
inode_mark, vfsmnt_mark, event_mask, data, data_type);
/* if we don't have enough info to send an event to userspace say no */
if (data_type != FSNOTIFY_EVENT_PATH)
return false;
/* sorry, fanotify only gives a damn about files and dirs */
if (!S_ISREG(path->dentry->d_inode->i_mode) &&
!S_ISDIR(path->dentry->d_inode->i_mode))
return false;
if (inode_mark && vfsmnt_mark) {
marks_mask = (vfsmnt_mark->mask | inode_mark->mask);
marks_ignored_mask = (vfsmnt_mark->ignored_mask | inode_mark->ignored_mask);
} else if (inode_mark) {
/*
* if the event is for a child and this inode doesn't care about
* events on the child, don't send it!
*/
if ((event_mask & FS_EVENT_ON_CHILD) &&
!(inode_mark->mask & FS_EVENT_ON_CHILD))
return false;
marks_mask = inode_mark->mask;
marks_ignored_mask = inode_mark->ignored_mask;
} else if (vfsmnt_mark) {
marks_mask = vfsmnt_mark->mask;
marks_ignored_mask = vfsmnt_mark->ignored_mask;
} else {
BUG();
}
if (S_ISDIR(path->dentry->d_inode->i_mode) &&
(marks_ignored_mask & FS_ISDIR))
return false;
if (event_mask & marks_mask & ~marks_ignored_mask)
return true;
return false;
}
static void fanotify_free_group_priv(struct fsnotify_group *group)
{
struct user_struct *user;
user = group->fanotify_data.user;
atomic_dec(&user->fanotify_listeners);
free_uid(user);
}
const struct fsnotify_ops fanotify_fsnotify_ops = {
.handle_event = fanotify_handle_event,
.should_send_event = fanotify_should_send_event,
.free_group_priv = fanotify_free_group_priv,
.free_event_priv = NULL,
.freeing_mark = NULL,
};