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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=jj3U7X8MKNKls/ApV8M/SIHb1JIZAnMiTroClnO0tIA=; b=oapDYuuU/ADkLZhYV/vdmR0TCQamUPn5wzFnkNfBWGTYUkZLrO0kbsZJSttV9iCzs+ ARrznO1pCDSCTNAAn1PbFRBKX8bag4Zpxpd1WrUfvruWDTXQwljuuCtN/91BUtopFoRz ent522FMBaNhRumJYzb9QKjrTiBcNOiEdUL5lrQqiIRA3fWL6PauTA8xZANiwfLy+9S3 8QwVfc/YY9H++zYlPM/4DbFRn/NwVdiB8GtVXL/lGpQXI/7gWz1IVPHfAxpb1DP3XKXC tMsOXsPOruhUmYBKItPxa0jwN1RFFCMWvnQkIS660PzoWOgsHtLtii2Mo27Ja4J67aB8 eQ3w== X-Gm-Message-State: AFqh2kpKkBdDkc1YK7LjLN25AeBnNOpYDfvsMCjtRTlUa/mEtRzis1aE flJtdpzQwyJmiiRZc5LjrwrmmqMngdmWgnGjeBgKlA== X-Google-Smtp-Source: AMrXdXvCH+b5CERll6lmutHDNNFVayFzrE8mxmTDaMWhbUfTorpT1Hz8BgbCIPElX7EZcrJh5yYvliMsy3/p4P0XO8Q= X-Received: by 2002:a02:c884:0:b0:39e:9d33:a47 with SMTP id m4-20020a02c884000000b0039e9d330a47mr271546jao.58.1673980603471; Tue, 17 Jan 2023 10:36:43 -0800 (PST) MIME-Version: 1.0 References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> In-Reply-To: From: Jann Horn Date: Tue, 17 Jan 2023 19:36:07 +0100 Message-ID: Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock To: Matthew Wilcox Cc: Suren Baghdasaryan , 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 Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: 9e4okptkch3ozcgtz7yinp9q9ozayg1e X-Rspamd-Queue-Id: 7B09B20006 X-HE-Tag: 1673980604-656991 X-HE-Meta: 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 KIEyCPua HtdE+qFUn3LO/XtqykcrWFumtx8iEhLyY9cupB0oV9uYTV9m+HsK2vZ7gVg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000006, 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 7:31 PM 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? I don't see that. When do_user_addr_fault() is calling vma_read_unlock(), there's no RCU read lock held, right?