On Thu, 29 Aug 2024 11:24:36 +0800 Chi Zhiling <[email protected]> wrote:
> From: Chi Zhiling <[email protected]> > > We found a null pointer accessing in tracefs[1], the reason is that > the variable 'ei_child' is set to LIST_POISON1, that means the list > was removed in eventfs_remove_rec. so when access the ei_child->is_freed, > the panic triggered. > > the linked list is protected by eventfs_mutex in eventfs_remove_rec, Only writes of the link list is protected by the mutex. Reads are not. > so when we access the list of ei_child in eventfs_iterate, we also need > a mutex_lock in there to avoid eventfs_remove_rec modify the list. Yes you hit a bug, but no this is *not* the solution! > Signed-off-by: Chi Zhiling <[email protected]> > --- > fs/tracefs/event_inode.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c > index 01e99e98457d..4895ed07376b 100644 > --- a/fs/tracefs/event_inode.c > +++ b/fs/tracefs/event_inode.c > @@ -642,6 +642,7 @@ static int eventfs_iterate(struct file *file, struct > dir_context *ctx) > /* Subtract the skipped entries above */ > c -= min((unsigned int)c, (unsigned int)ei->nr_entries); > > + mutex_unlock(&eventfs_mutex); This list is protected by SRCU (hence the name of the iterator), if you need to add a mutex around it, something else is broken. > list_for_each_entry_srcu(ei_child, &ei->children, list, > srcu_read_lock_held(&eventfs_srcu)) { > > @@ -659,9 +660,12 @@ static int eventfs_iterate(struct file *file, struct > dir_context *ctx) > > ino = eventfs_dir_ino(ei_child); > > - if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR)) > + if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR)) { > + mutex_unlock(&eventfs_mutex); > goto out_dec; > + } > } > + mutex_unlock(&eventfs_mutex); > ret = 1; > out: > srcu_read_unlock(&eventfs_srcu, idx); The real fix is: diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c index 01e99e98457d..8705c77a9e75 100644 --- a/fs/tracefs/event_inode.c +++ b/fs/tracefs/event_inode.c @@ -862,7 +862,7 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, int level) list_for_each_entry(ei_child, &ei->children, list) eventfs_remove_rec(ei_child, level + 1); - list_del(&ei->list); + list_del_rcu(&ei->list); free_ei(ei); } Can you test that and let me know if it fixes your issue. I'll just go ahead and apply it as it is an obvious bug. -- Steve
