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Mon, 10 Mar 2025 14:40:35 +0800 Message-ID: Date: Mon, 10 Mar 2025 14:40:35 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released To: Peter Xu , jimsiak CC: , , , , References: <79375b71-db2e-3e66-346b-254c90d915e2@cslab.ece.ntua.gr> <20250307072133.3522652-1-tujinjiang@huawei.com> <46ac83f7-d3e0-b667-7352-d853938c9fc9@huawei.com> From: Jinjiang Tu In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.179.13] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To kwepemo200002.china.huawei.com (7.202.195.209) X-Stat-Signature: egeba8znc9g1xzxi7bu4qt8awx3euy75 X-Rspam-User: X-Rspamd-Queue-Id: 6CF7E140008 X-Rspamd-Server: rspam01 X-HE-Tag: 1741588843-510442 X-HE-Meta: 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 ac9zzpIn iKRlXi8FX91zR0pmR7IPrgwI4ODhSNFxi1yCjQy9s6/GwCNLogU4iDwdgBqztYvRG6HYaTS6yhkyyVskztYcCFkbrzMlQAzxq+Cxk6DnXET493TCs2vk1O4helPDf/Gqe956EUYm0M3SoRhBA4B7OrHqBiwe3FEAnT6xlthWWFzhDAvr8GN4GKssHeNIqHjmZs2NVR0svVynlB8OJ9GlOnO0eH9Z7KfM6GDzGlVDreUO/WE5X6qkJq7KJLus2200FV06wkhzBE7+2oRg6o7xt/+9DiOJ5RD3/3xbGJlcOwA7ZDYrvSX8auLA/Gg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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？ commit 656710a60e3693911bee3a355d2f2bbae3faba33 Author: Andrea Arcangeli Date: Fri Sep 8 16:12:42 2017 -0700 userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS > > Thanks, >