Re: [PATCH v4 net] net: page_pool: fix UAF in __page_pool_release_netmem_dma on xa_cmpxchg race

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 4 Aug 2026 20:50:46 -0700 Mina Almasry wrote:
> > Not sure this is a good idea? scrub is trying to touch just the DMA
> > mapping, right? It shouldn't try to update the page itself because
> > it has no reference to the page, the page may get freed in parallel.
> > Hopefully DMA unmap on a freed page is legal..  
> 
> Scrub actually frees the page in the loop in
> page_pool_empty_ring(pool), so it's not true AFAIU that it 'shouldn't
> try to update the page'.
> 
> My mental model (roughly) is that the pp has a ref and that single
> last ref can be dropped in the page_pool_destroy() path or the
> page_pool_put_netmem() path, but the ref can't be dropped twice and
> the same goes for dma unmapping.
> 
> Now that i look closer at the code I have no idea why we didn't 'just'
> put the dma-unmapping inside of page_pool_return_netmem(). That code
> path is already common between scrub() and put_netmem() and does the
> synchronization between these 2 paths.

from memory so likely wrong -
ring scrub is freeing pages from the cache
DMA unamp is clearing _all_ mappings, including actively on used pages
the are different sets