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([2001:b07:6468:f312:56e1:adff:fed9:caf0]) by smtp.gmail.com with ESMTPSA id 205sm17357857wmb.3.2019.11.29.10.27.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 29 Nov 2019 10:27:33 -0800 (PST) Subject: Re: libcap vs libcap-ng mess To: "Dr. David Alan Gilbert" References: <20191128190408.GC3294@work-vm> <20191129093409.GB2260471@redhat.com> <98520a07-cf5d-a2a9-cfa4-944839b94c7c@redhat.com> <20191129180103.GA2840@work-vm> <1e59f880-1a62-3230-c56a-533f2b797525@redhat.com> <20191129182021.GD2837@work-vm> From: Paolo Bonzini Message-ID: Date: Fri, 29 Nov 2019 19:27:32 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20191129182021.GD2837@work-vm> Content-Language: en-US X-MC-Unique: RStfoyO-Nw-jJDumBqSStA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: thuth@redhat.com, =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , qemu-devel@nongnu.org, vgoyal@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 29/11/19 19:20, Dr. David Alan Gilbert wrote: > * Paolo Bonzini (pbonzini@redhat.com) wrote: >> On 29/11/19 19:01, Dr. David Alan Gilbert wrote: >>>> It's not entirely trivial because fsdev-proxy-helper wants to keep the >>>> effective set and clear the permitted set; in libcap-ng you can only >> ^^^^^ >> >> (Wrong, this is "modify" the permitted set. The permitted set is >> already cleared by setresuid/setresgid). >> >>>> apply both sets at once, and you cannot choose only one of them in >>>> capng_clear/capng_get_caps_process. But it's doable, I'll take a look. >>> I'm having some difficulties making the same conversion for virtiofsd; >>> all it wants to do is drop (and later recover) CAP_FSETID >>> from it's effective set; so I'm calling capng_get_caps_process >>> (it used to be cap_get_proc). While libcap survives just using the >>> capget syscall, libcap-ng wants to read /proc//status - and >>> that's a problem because we're in a sandbox without /proc mounted >>> at that point. >> >> The state of libcap-ng persists after capng_apply. So you can just call >> capng_update({CAP_ADD,CAP_DROP}) followed by capng_apply. > > But the internal state needs initialising doesn't it? So that when you > capng_update it tweaks a set that was originally read from somewhere? > (and that's per-thread?) Yes, it's per thread. The state can be built from capng_clear/capng_get_caps_process + capng_update, and left in there forever. There is also capng_save_state/capng_restore_state which, as far as I can see from the sources, can be used across threads. >> Does virtiofsd have to do uid/gid dances like virtfs-proxy-helper? > > It looks like it; I can see setresuid calls to save and restore > euid/egid. Ok, then perhaps you can take a look at my virtfs-proxy-helper patch. The important part is that after setresuid/setresgid PERM=EFF if uid=0/gid=0 and PERM=0 otherwise. Paolo