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([2601:646:c200:7429:11a8:656c:46d0:f835]) by smtp.gmail.com with ESMTPSA id g6-v6sm19661256pfb.11.2018.10.01.07.28.35 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 01 Oct 2018 07:28:35 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (1.0) Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution From: Andy Lutomirski X-Mailer: iPhone Mail (16A366) In-Reply-To: <20181001054246.gfinmx3api7kjhmc@ryuk> Date: Mon, 1 Oct 2018 07:28:34 -0700 Cc: Jann Horn , "Eric W. Biederman" , jlayton@kernel.org, Bruce Fields , Al Viro , Arnd Bergmann , shuah@kernel.org, David Howells , Andy Lutomirski , christian@brauner.io, Tycho Andersen , kernel list , linux-fsdevel@vger.kernel.org, linux-arch , linux-kselftest@vger.kernel.org, dev@opencontainers.org, containers@lists.linux-foundation.org, Linux API Content-Transfer-Encoding: quoted-printable Message-Id: References: <20180929103453.12025-1-cyphar@cyphar.com> <20180929131534.24472-1-cyphar@cyphar.com> <20181001054246.gfinmx3api7kjhmc@ryuk> To: Aleksa Sarai Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> Currently most container runtimes try to do this resolution in >>> userspace[1], causing many potential race conditions. In addition, the >>> "obvious" alternative (actually performing a {ch,pivot_}root(2)) >>> requires a fork+exec which is *very* costly if necessary for every >>> filesystem operation involving a container. >>=20 >> Wait. fork() I understand, but why exec? And actually, you don't need >> a full fork() either, clone() lets you do this with some process parts >> shared. And then you also shouldn't need to use SCM_RIGHTS, just keep >> the file descriptor table shared. And why chroot()/pivot_root(), >> wouldn't you want to use setns()? >=20 > You're right about this -- for C runtimes. In Go we cannot do a raw > clone() or fork() (if you do it manually with RawSyscall you'll end with > broken runtime state). So you're forced to do fork+exec (which then > means that you can't use CLONE_FILES and must use SCM_RIGHTS). Same goes > for CLONE_VFORK. I must admit that I=E2=80=99m not very sympathetic to the argument that =E2=80= =9CGo=E2=80=99s runtime model is incompatible with the simpler solution.=E2=80= =9D Anyway, it occurs to me that the real problem is that setns() and chroot() a= re both overkill for this use case. What=E2=80=99s needed is to start your w= alk from /proc/pid-in-container/root, with two twists: 1. Do the walk as though rooted at a directory. This is basically just your A= T_THIS_ROOT, but the footgun is avoided because the dirfd you use is from a f= oreign namespace, and, except for symlinks to absolute paths, no amount of .= . racing is going to escape the *namespace*. 2. Avoid /proc. It=E2=80=99s not just the *links* =E2=80=94 you really don=E2= =80=99t want to walk into /proc/self. *Maybe* procfs is already careful enou= gh when mounted in a namespace?