Re: Does have race between __mark_inode_dirty() and evict()

[Jan Kara <jack@suse.cz>]

Hi!

On Mon 20-04-20 14:23:19, Xiaoguang Wang wrote:
> Recently we run into a NULL pointer dereference panic in our internal 4.9
> kernel it panics because inode->i_wb has become zero in
> wbc_attach_and_unlock_inode(), and by crash tools analysis, inode's
> dirtied_when is zero, but dirtied_time_when is not zero, seems that this
> inode has been used after free. Looking into both 4.9 and upstream codes,
> seems that there maybe a race:
> 
> __mark_inode_dirty(...)
> {
>     spin_lock(&inode->i_lock);
>     ...
>     if (inode->i_state & I_FREEING)
>         goto out_unlock_inode;
>     ...
>     if (!was_dirty) {
>         struct bdi_writeback *wb;
>         struct list_head *dirty_list;
>         bool wakeup_bdi = false;
> 
>         wb = locked_inode_to_wb_and_lock_list(inode);
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>        this function will unlock inode->i_ilock firstly and then relock,
>        but once the inode->i_ilock is unlocked, evict() may run in, set
>        I_FREEING flag, and free the inode, and later
>        locked_inode_to_wb_and_lock_list relocks inode->i_ilock again, but
>        will not check the I_FREEING flag again, so the use after free for
>        this inode would happen.

If someone is calling __mark_inode_dirty(), he should be holding inode
reference (either directly or indirectly through having dentry reference,
file open, ...) which prevents the scenario you suggest.

But I've just recently tracked down a very similarly looking issue reported
to me. I've submitted patches to fix it here [1].

[1] https://lore.kernel.org/linux-ext4/20200421085445.5731-1-jack@suse.cz

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR