Re: [PATCH] eventfs: fix a null pointer access in eventfs_iterate

[Chi Zhiling]

On 2024/9/4 03:51, Steven Rostedt wrote:
> On Thu, 29 Aug 2024 11:24:36 +0800
> Chi Zhiling <chizhiling@xxxxxxx> wrote:
>
> > From: Chi Zhiling <chizhiling@xxxxxxxxxx>
> >
> > 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 <chizhiling@xxxxxxxxxx>
> > ---
> >   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.
Okay, I've tested it and I haven't seen any errors.