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Tsirkin" Cc: virtualization@lists.linux.dev, kvm@vger.kernel.org, netdev@vger.kernel.org, jasowang@redhat.com, michael.christie@oracle.com, pbonzini@redhat.com, stefanha@redhat.com, eperezma@redhat.com, joao.m.martins@oracle.com, joe.jin@oracle.com, si-wei.liu@oracle.com, linux-kernel@vger.kernel.org References: <20250403063028.16045-1-dongli.zhang@oracle.com> <20250403063028.16045-10-dongli.zhang@oracle.com> <20250414123119-mutt-send-email-mst@kernel.org> <20250414143039-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Dongli Zhang In-Reply-To: <20250414143039-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BL1P223CA0024.NAMP223.PROD.OUTLOOK.COM (2603:10b6:208:2c4::29) To DS7PR10MB7129.namprd10.prod.outlook.com (2603:10b6:8:e6::5) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR10MB7129:EE_|PH8PR10MB6526:EE_ X-MS-Office365-Filtering-Correlation-Id: 334138c2-e164-4351-78e7-08dd7b965719 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016; 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Tsirkin wrote: > On Mon, Apr 14, 2025 at 09:52:04AM -0700, Dongli Zhang wrote: >> Hi Michael, >> >> On 4/14/25 9:32 AM, Michael S. Tsirkin wrote: >>> On Wed, Apr 02, 2025 at 11:29:54PM -0700, Dongli Zhang wrote: >>>> Since long time ago, the only user of vq->log is vhost-net. The concern is >>>> to add support for more devices (i.e. vhost-scsi or vsock) may reveals >>>> unknown issue in the vhost API. Add a WARNING. >>>> >>>> Suggested-by: Joao Martins >>>> Signed-off-by: Dongli Zhang >>> >>> >>> Userspace can trigger this I think, this is a problem since >>> people run with reboot on warn. >> >> I think it will be a severe kernel bug (page fault) if userspace can trigger this. >> >> If (*log_num >= vq->dev->iov_limit), the next line will lead to an out-of-bound >> memory access: >> >> log[*log_num].addr = vhost64_to_cpu(vq, desc.addr); >> >> I could not propose a case to trigger the WARNING from userspace. Would you mind >> helping explain if that can happen? > > Oh I see. the commit log made me think this is an actual issue, > not a debugging aid just in case. > > >>> Pls grammar issues in comments... I don't think so. >> >> I did an analysis of code and so far I could not identify any case to trigger >> (*log_num >= vq->dev->iov_limit). >> >> The objective of the patch is to add a WARNING to double confirm the case won't >> happen. >> >> Regarding "I don't think so", would you mean we don't need this patch/WARNING >> because the code is robust enough? >> >> Thank you very much! >> >> Dongli Zhang > > > Let me clarify the comment is misleading. > All it has to say is: > > /* Let's make sure we are not out of bounds. */ > BUG_ON(*log_num >= vq->dev->iov_limit); This is a critical path only during Live Migration is in progress. So far I didn't encounter any issue for either vhost-net or vhost-scsi. That's why I used WARN_ON_ONCE() instead of a BUG_ON(). I agree with your point on "unnecessary pointer chasing on critical path", I am going to remove this patch in the next version. Thank you very much for feedback and suggestion! Dongli Zhang > > at the same time, this is unnecessary pointer chasing > on critical path, and I don't much like it that we are > making an assumption about array size here. > > If you strongly want to do it, you must document it near > get_indirect: > @log - array of size at least vq->dev->iov_limit >