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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHr04-009vFr-26; Tue, 17 Jan 2023 18:48:00 +0000 Date: Tue, 17 Jan 2023 18:48:00 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Jann Horn , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 7A73D8001A X-Rspam-User: X-Stat-Signature: obypymwhb5qj6akfkmdfsp3g1k5utrjt X-HE-Tag: 1673981310-371448 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jan 17, 2023 at 10:36:42AM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 17, 2023 at 10:31 AM Matthew Wilcox wrote: > > > > On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote: > > > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn wrote: > > > > > > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > > > > > rw_semaphore is a sizable structure of 40 bytes and consumes > > > > > considerable space for each vm_area_struct. However vma_lock has > > > > > two important specifics which can be used to replace rw_semaphore > > > > > with a simpler structure: > > > > [...] > > > > > static inline void vma_read_unlock(struct vm_area_struct *vma) > > > > > { > > > > > - up_read(&vma->vm_lock->lock); > > > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > > > } > > > > > > > > I haven't properly reviewed this, but this bit looks like a > > > > use-after-free because you're accessing the vma after dropping your > > > > reference on it. You'd have to first look up the vma->vm_mm, then do > > > > the atomic_dec_and_test(), and afterwards do the wake_up() without > > > > touching the vma. Or alternatively wrap the whole thing in an RCU > > > > read-side critical section if the VMA is freed with RCU delay. > > > > > > vm_lock->count does not control the lifetime of the VMA, it's a > > > counter of how many readers took the lock or it's negative if the lock > > > is write-locked. > > > > Yes, but ... > > > > Task A: > > atomic_dec_and_test(&vma->vm_lock->count) > > Task B: > > munmap() > > write lock > > free VMA > > synchronize_rcu() > > VMA is really freed > > wake_up(&vma->vm_mm->vma_writer_wait); > > > > ... vma is freed. > > > > Now, I think this doesn't occur. I'm pretty sure that every caller of > > vma_read_unlock() is holding the RCU read lock. But maybe we should > > have that assertion? > > Yep, that's what this patch is doing > https://lore.kernel.org/all/20230109205336.3665937-27-surenb@google.com/ > by calling vma_assert_no_reader() from __vm_area_free(). That's not enough though. Task A still has a pointer to vma after it has called atomic_dec_and_test(), even after vma has been freed by Task B, and before Task A dereferences vma->vm_mm.