Re: [PATCH v5 0/3] mm/vmalloc: free unused pages on vrealloc() shrink

[Uladzislau Rezki <urezki@gmail.com>]

On Sun, Mar 22, 2026 at 12:48:28PM +0000, Alice Ryhl wrote:
> On Sat, Mar 21, 2026 at 01:45:35PM +0530, Shivam Kalra wrote:
> > On 17/03/26 13:47, Shivam Kalra via B4 Relay wrote:
> > 3. Fixing a /proc/vmallocinfo race condition
> > `show_numa_info()` iterates over `v->nr_pages`. During a shrink,
> > modifying `nr_pages` and NULL-ing out the page pointers concurrently
> > could cause a reader to dereference a NULL page pointer.
> > Plan: I'll update the reader to use `READ_ONCE(v->nr_pages)`, and have
> > the shrink path do a `WRITE_ONCE(vm->nr_pages, new_nr_pages)` before
> > freeing the pages. This guarantees that concurrent readers either see
> > the old count with valid pages or the new, smaller count.
> 
> This doesn't fix the race. Consider this:
> 
> nr < vm->nr_pages == true
> 		vm->nr_pages = nr
> 		free vm->pages[nr]
> page_to_nid(v->pages[nr]) // UAF
> 
> perhaps changing vm->nr_pages should happen under the vn->busy.lock
> spinlock? show_numa_info() is called under that lock too.
> 
vn->busy.lock protects VA in a busy tree. So if you update the nr_pages
of given VA you should hold the lock of the node VA belongs to.

--
Uladzislau Rezki