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[74.65.150.180]) by smtp.gmail.com with ESMTPSA id k19sm8094710qta.90.2021.06.29.13.28.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Jun 2021 13:28:26 -0700 (PDT) Reply-To: dwalsh@redhat.com Subject: Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS_RESOURCE To: Vivek Goyal , Casey Schaufler Cc: "Dr. David Alan Gilbert" , "Schaufler, Casey" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "viro@zeniv.linux.org.uk" , "virtio-fs@redhat.com" , "berrange@redhat.com" , linux-security-module , "selinux@vger.kernel.org" References: <20210628131708.GA1803896@redhat.com> <1b446468-dcf8-9e21-58d3-c032686eeee5@redhat.com> <5d8f033c-eba2-7a8b-f19a-1005bbb615ea@schaufler-ca.com> <20210629152007.GC5231@redhat.com> <78663f5c-d2fd-747a-48e3-0c5fd8b40332@schaufler-ca.com> <20210629173530.GD5231@redhat.com> From: Daniel Walsh Organization: Red Hat Message-ID: Date: Tue, 29 Jun 2021 16:28:24 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210629173530.GD5231@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 6/29/21 13:35, Vivek Goyal wrote: > On Tue, Jun 29, 2021 at 09:13:48AM -0700, Casey Schaufler wrote: >> On 6/29/2021 8:20 AM, Vivek Goyal wrote: >>> On Tue, Jun 29, 2021 at 07:38:15AM -0700, Casey Schaufler wrote: >>> >>> [..] >>>>>>>> User xattrs are less protected than security xattrs. You are exposing the >>>>>>>> security xattrs on the guest to the possible whims of a malicious, unprivileged >>>>>>>> actor on the host. All it needs is the right UID. >>>>>>> Yep, we realise that; but when you're mainly interested in making sure >>>>>>> the guest can't attack the host, that's less worrying. >>>>>> That's uncomfortable. >>>>> Why exactly? >>>> If a mechanism is designed with a known vulnerability you >>>> fail your validation/evaluation efforts. >>> We are working with the constraint that shared directory should not be >>> accessible to unpriviliged users on host. And with that constraint, what >>> you are referring to is not a vulnerability. >> Sure, that's quite reasonable for your use case. It doesn't mean >> that the vulnerability doesn't exist, it means you've mitigated it. >> >> >>>> Your mechanism is >>>> less general because other potential use cases may not be >>>> as cavalier about the vulnerability. >>> Prefixing xattrs with "user.virtiofsd" is just one of the options. >>> virtiofsd has the capability to prefix "trusted.virtiofsd" as well. >>> We have not chosen that because we don't want to give it CAP_SYS_ADMIN. >>> >>> So other use cases which don't like prefixing "user.virtiofsd", can >>> give CAP_SYS_ADMIN and work with it. >>> >>>> I think that you can >>>> approach this differently, get a solution that does everything >>>> you want, and avoid the known problem. >>> What's the solution? Are you referring to using "trusted.*" instead? But >>> that has its own problem of giving CAP_SYS_ADMIN to virtiofsd. >> I'm coming to the conclusion that xattr namespaces, analogous >> to user namespaces, are the correct solution. They generalize >> for multiple filesystem and LSM use cases. The use of namespaces >> is well understood, especially in the container community. It >> looks to me as if it would address your use case swimmingly. > Even if xattrs were namespaced, I am not sure it solves the issue > of unpriviliged UID being able to modify security xattrs of file. > If it happens to be correct UID, it should be able to spin up a > user namespace and modify namespaced xattrs? > > Anyway, once namespaced xattrs are available, I will gladly make use > of it. But that probably should not be a blocker for this patch. > > Vivek > All this conversation is great, and I look forward to a better solution, but if we go back to the patch, it was to fix an issue where the kernel is requiring CAP_SYS_ADMIN for writing user Xattrs on link files and other special files. The documented reason for this is to prevent the users from using XATTRS to avoid quota. The CAP_SYS_RESOURCE capability is denfined to allow processes with this capability to ignore quota. This PR allows processes with CAP_SYS_RESOURCE to create user Xattrs. To me this makes sense. Is there any argument against this?