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Mon, 10 Mar 2025 11:50:38 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFie+mzm2Bw8VvHKP/u+MGW8Iw61YPMj501OCpii1PEfvhFOD2xnDZkzVVnlM30dQ3KOy/+cA== X-Received: by 2002:a05:6214:21cb:b0:6e8:9fcb:5f7d with SMTP id 6a1803df08f44-6e9006913cdmr229708106d6.36.1741632638222; Mon, 10 Mar 2025 11:50:38 -0700 (PDT) Received: from x1.local ([85.131.185.92]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6e8f7170958sm61093186d6.108.2025.03.10.11.50.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Mar 2025 11:50:37 -0700 (PDT) Date: Mon, 10 Mar 2025 14:50:34 -0400 From: Peter Xu To: Jinjiang Tu Cc: jimsiak , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, linux-mm@kvack.org, wangkefeng.wang@huawei.com Subject: Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released Message-ID: References: <79375b71-db2e-3e66-346b-254c90d915e2@cslab.ece.ntua.gr> <20250307072133.3522652-1-tujinjiang@huawei.com> <46ac83f7-d3e0-b667-7352-d853938c9fc9@huawei.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Mar 10, 2025 at 02:40:35PM +0800, Jinjiang Tu wrote: > > 在 2025/3/8 6:41, Peter Xu 写道: > > On Fri, Mar 07, 2025 at 03:11:09PM +0200, jimsiak wrote: > > > Hi, > > > > > > From my side, I managed to avoid the freezing of processes with the > > > following change in function userfaultfd_release() in file fs/userfaultfd.c > > > (https://elixir.bootlin.com/linux/v5.13/source/fs/userfaultfd.c#L842): > > > > > > I moved the following command from line 851: > > > WRITE_ONCE(ctx->released, true); > > > (https://elixir.bootlin.com/linux/v5.13/source/fs/userfaultfd.c#L851) > > > > > > to line 905, that is exactly before the functions returns 0. > > > > > > That simple workaround worked for my use case but I am far from sure that is > > > a correct/sufficient fix for the problem at hand. > > Updating the field after userfaultfd_ctx_put() might mean UAF, afaict. > > > > Maybe it's possible to remove ctx->released but only rely on the mmap write > > lock. However that'll need some closer look and more thoughts. > > > > To me, the more straightforward way to fix it is to use the patch I > > mentioned in the other email: > > > > https://lore.kernel.org/all/ZLmT3BfcmltfFvbq@x1n/ > > > > Or does it mean it didn't work at all? > > This patch works for me. mlock() syscall calls GUP with FOLL_UNLOCKABLE and > allows to release mmap lock and retry. > > But other GUP call without FOLL_UNLOCKABLE will return VM_FAULT_SIGBUS, > is it a regression for the below commit？ Do you have an explicit reproducer / use case of such? AFAIU, below commit should only change it from SIGBUS to NOPAGE when "released" is set. I don't see how it can regress on !FOLL_UNLOCKABLE. Thanks, > > commit 656710a60e3693911bee3a355d2f2bbae3faba33 > Author: Andrea Arcangeli > Date: Fri Sep 8 16:12:42 2017 -0700 > > userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS > > > > > Thanks, > > > -- Peter Xu