Re: [PATCH RESEND] locking/ww_mutex: Adjust to lockdep nest_lock requirements

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 02, 2024 at 02:56:11PM +0200, Thomas Hellström wrote:
> When using mutex_acquire_nest() with a nest_lock, lockdep refcounts the
> number of acquired lockdep_maps of mutexes of the same class, and also
> keeps a pointer to the first acquired lockdep_map of a class. That pointer
> is then used for various comparison-, printing- and checking purposes,
> but there is no mechanism to actively ensure that lockdep_map stays in
> memory. Instead, a warning is printed if the lockdep_map is freed and
> there are still held locks of the same lock class, even if the lockdep_map
> itself has been released.
> 
> In the context of WW/WD transactions that means that if a user unlocks
> and frees a ww_mutex from within an ongoing ww transaction, and that
> mutex happens to be the first ww_mutex grabbed in the transaction,
> such a warning is printed and there might be a risk of a UAF.

I'm assuming you actually hit this?

Anyway, work around seems sane enough, thanks!