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Sun, 22 Mar 2026 07:32:14 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Sun, 22 Mar 2026 15:32:12 +0100 To: Alice Ryhl Cc: Shivam Kalra , Andrew Morton , Uladzislau Rezki , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Danilo Krummrich Subject: Re: [PATCH v5 0/3] mm/vmalloc: free unused pages on vrealloc() shrink Message-ID: References: <20260317-vmalloc-shrink-v5-0-bbfbf54c5265@zohomail.in> <03262674-2df0-40d8-b411-4735c6f43770@zohomail.in> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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