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Thu, 22 Oct 2020 15:24:42 -0400 (EDT) Date: Thu, 22 Oct 2020 15:24:42 -0400 From: Vivek Goyal To: "Dr. David Alan Gilbert" Subject: Re: [PATCH v3] virtiofsd: add container-friendly -o sandbox=chroot option Message-ID: <20201022192442.GE512900@redhat.com> References: <20201008085534.16070-1-stefanha@redhat.com> <20201022191954.GK2951@work-vm> MIME-Version: 1.0 In-Reply-To: <20201022191954.GK2951@work-vm> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vgoyal@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=vgoyal@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/22 08:33:10 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: vromanso@redhat.com, Daniel Walsh , qemu-devel@nongnu.org, rmohr@redhat.com, virtio-fs@redhat.com, Al Viro , Stefan Hajnoczi , mpatel@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Oct 22, 2020 at 08:19:54PM +0100, Dr. David Alan Gilbert wrote: > * Stefan Hajnoczi (stefanha@redhat.com) wrote: > > virtiofsd cannot run in a container because CAP_SYS_ADMIN is required to > > create namespaces. > > > > Introduce a weaker sandbox mode that is sufficient in container > > environments because the container runtime already sets up namespaces. > > Use chroot to restrict path traversal to the shared directory. > > > > virtiofsd loses the following: > > > > 1. Mount namespace. The process chroots to the shared directory but > > leaves the mounts in place. Seccomp rejects mount(2)/umount(2) > > syscalls. > > > > 2. Pid namespace. This should be fine because virtiofsd is the only > > process running in the container. > > > > 3. Network namespace. This should be fine because seccomp already > > rejects the connect(2) syscall, but an additional layer of security > > is lost. Container runtime-specific network security policies can be > > used drop network traffic (except for the vhost-user UNIX domain > > socket). > > > > Signed-off-by: Stefan Hajnoczi > > I've just tripped over another case where this probably helps (but not > yet tested...); pivot_root doesn't work if your current / isn't a > mountpoint - so you can't currently run the existing virtiofsd inside > a chroot. Can we avoid that issue simply by doing a bind mount of directory before chroot(). Vivek > > (pivot_root is awful for telling you this - it has 6 different manpage > listed reasons it might return EINVAL and leaves you to figure out how > you offended it). > > Dave > > > --- > > v3: > > * Rebased onto David Gilbert's latest migration & virtiofsd pull > > request > > > > tools/virtiofsd/helper.c | 8 +++++ > > tools/virtiofsd/passthrough_ll.c | 57 ++++++++++++++++++++++++++++++-- > > docs/tools/virtiofsd.rst | 32 ++++++++++++++---- > > 3 files changed, 88 insertions(+), 9 deletions(-) > > > > diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c > > index 85770d63f1..2e181a49b5 100644 > > --- a/tools/virtiofsd/helper.c > > +++ b/tools/virtiofsd/helper.c > > @@ -166,6 +166,14 @@ void fuse_cmdline_help(void) > > " enable/disable readirplus\n" > > " default: readdirplus except with " > > "cache=none\n" > > + " -o sandbox=namespace|chroot\n" > > + " sandboxing mode:\n" > > + " - namespace: mount, pid, and net\n" > > + " namespaces with pivot_root(2)\n" > > + " into shared directory\n" > > + " - chroot: chroot(2) into shared\n" > > + " directory (use in containers)\n" > > + " default: namespace\n" > > " -o timeout= I/O timeout (seconds)\n" > > " default: depends on cache= option.\n" > > " -o writeback|no_writeback enable/disable writeback cache\n" > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > > index ff53df4451..5b9064278a 100644 > > --- a/tools/virtiofsd/passthrough_ll.c > > +++ b/tools/virtiofsd/passthrough_ll.c > > @@ -137,8 +137,14 @@ enum { > > CACHE_ALWAYS, > > }; > > > > +enum { > > + SANDBOX_NAMESPACE, > > + SANDBOX_CHROOT, > > +}; > > + > > struct lo_data { > > pthread_mutex_t mutex; > > + int sandbox; > > int debug; > > int writeback; > > int flock; > > @@ -163,6 +169,12 @@ struct lo_data { > > }; > > > > static const struct fuse_opt lo_opts[] = { > > + { "sandbox=namespace", > > + offsetof(struct lo_data, sandbox), > > + SANDBOX_NAMESPACE }, > > + { "sandbox=chroot", > > + offsetof(struct lo_data, sandbox), > > + SANDBOX_CHROOT }, > > { "writeback", offsetof(struct lo_data, writeback), 1 }, > > { "no_writeback", offsetof(struct lo_data, writeback), 0 }, > > { "source=%s", offsetof(struct lo_data, source), 0 }, > > @@ -2660,6 +2672,41 @@ static void setup_capabilities(char *modcaps_in) > > pthread_mutex_unlock(&cap.mutex); > > } > > > > +/* > > + * Use chroot as a weaker sandbox for environments where the process is > > + * launched without CAP_SYS_ADMIN. > > + */ > > +static void setup_chroot(struct lo_data *lo) > > +{ > > + lo->proc_self_fd = open("/proc/self/fd", O_PATH); > > + if (lo->proc_self_fd == -1) { > > + fuse_log(FUSE_LOG_ERR, "open(\"/proc/self/fd\", O_PATH): %m\n"); > > + exit(1); > > + } > > + > > + /* > > + * Make the shared directory the file system root so that FUSE_OPEN > > + * (lo_open()) cannot escape the shared directory by opening a symlink. > > + * > > + * The chroot(2) syscall is later disabled by seccomp and the > > + * CAP_SYS_CHROOT capability is dropped so that tampering with the chroot > > + * is not possible. > > + * > > + * However, it's still possible to escape the chroot via lo->proc_self_fd > > + * but that requires first gaining control of the process. > > + */ > > + if (chroot(lo->source) != 0) { > > + fuse_log(FUSE_LOG_ERR, "chroot(\"%s\"): %m\n", lo->source); > > + exit(1); > > + } > > + > > + /* Move into the chroot */ > > + if (chdir("/") != 0) { > > + fuse_log(FUSE_LOG_ERR, "chdir(\"/\"): %m\n"); > > + exit(1); > > + } > > +} > > + > > /* > > * Lock down this process to prevent access to other processes or files outside > > * source directory. This reduces the impact of arbitrary code execution bugs. > > @@ -2667,8 +2714,13 @@ static void setup_capabilities(char *modcaps_in) > > static void setup_sandbox(struct lo_data *lo, struct fuse_session *se, > > bool enable_syslog) > > { > > - setup_namespaces(lo, se); > > - setup_mounts(lo->source); > > + if (lo->sandbox == SANDBOX_NAMESPACE) { > > + setup_namespaces(lo, se); > > + setup_mounts(lo->source); > > + } else { > > + setup_chroot(lo); > > + } > > + > > setup_seccomp(enable_syslog); > > setup_capabilities(g_strdup(lo->modcaps)); > > } > > @@ -2815,6 +2867,7 @@ int main(int argc, char *argv[]) > > struct fuse_session *se; > > struct fuse_cmdline_opts opts; > > struct lo_data lo = { > > + .sandbox = SANDBOX_NAMESPACE, > > .debug = 0, > > .writeback = 0, > > .posix_lock = 0, > > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst > > index 7ecee49834..65f8e76569 100644 > > --- a/docs/tools/virtiofsd.rst > > +++ b/docs/tools/virtiofsd.rst > > @@ -17,13 +17,24 @@ This program is designed to work with QEMU's ``--device vhost-user-fs-pci`` > > but should work with any virtual machine monitor (VMM) that supports > > vhost-user. See the Examples section below. > > > > -This program must be run as the root user. Upon startup the program will > > -switch into a new file system namespace with the shared directory tree as its > > -root. This prevents "file system escapes" due to symlinks and other file > > -system objects that might lead to files outside the shared directory. The > > -program also sandboxes itself using seccomp(2) to prevent ptrace(2) and other > > -vectors that could allow an attacker to compromise the system after gaining > > -control of the virtiofsd process. > > +This program must be run as the root user. The program drops privileges where > > +possible during startup although it must be able to create and access files > > +with any uid/gid: > > + > > +* The ability to invoke syscalls is limited using seccomp(2). > > +* Linux capabilities(7) are dropped. > > + > > +In "namespace" sandbox mode the program switches into a new file system > > +namespace and invokes pivot_root(2) to make the shared directory tree its root. > > +A new pid and net namespace is also created to isolate the process. > > + > > +In "chroot" sandbox mode the program invokes chroot(2) to make the shared > > +directory tree its root. This mode is intended for container environments where > > +the container runtime has already set up the namespaces and the program does > > +not have permission to create namespaces itself. > > + > > +Both sandbox modes prevent "file system escapes" due to symlinks and other file > > +system objects that might lead to files outside the shared directory. > > > > Options > > ------- > > @@ -69,6 +80,13 @@ Options > > * readdirplus|no_readdirplus - > > Enable/disable readdirplus. The default is ``readdirplus``. > > > > + * sandbox=namespace|chroot - > > + Sandbox mode: > > + - namespace: Create mount, pid, and net namespaces and pivot_root(2) into > > + the shared directory. > > + - chroot: chroot(2) into shared directory (use in containers). > > + The default is "namespace". > > + > > * source=PATH - > > Share host directory tree located at PATH. This option is required. > > > > -- > > 2.26.2 > > > -- > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK