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Wed, 22 Jul 2020 19:03:20 +0000 (UTC) Date: Wed, 22 Jul 2020 20:03:18 +0100 From: "Dr. David Alan Gilbert" To: Stefan Hajnoczi Subject: Re: [PATCH for-5.1 2/3] virtiofsd: add container-friendly -o chroot sandboxing option Message-ID: <20200722190318.GM2660@work-vm> References: <20200722130206.224898-1-stefanha@redhat.com> <20200722130206.224898-3-stefanha@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200722130206.224898-3-stefanha@redhat.com> User-Agent: Mutt/1.14.5 (2020-06-23) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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=207.211.31.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/22 00:40:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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: virtio-fs@redhat.com, rmohr@redhat.com, qemu-devel@nongnu.org, vromanso@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Stefan Hajnoczi (stefanha@redhat.com) wrote: > virtiofsd cannot run in an unprivileged container because CAP_SYS_ADMIN > is required to create namespaces. > > Introduce a weaker sandbox 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 > --- > tools/virtiofsd/helper.c | 3 +++ > tools/virtiofsd/passthrough_ll.c | 44 ++++++++++++++++++++++++++++++-- > 2 files changed, 45 insertions(+), 2 deletions(-) > > diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c > index 3105b6c23a..7421c9ca1a 100644 > --- a/tools/virtiofsd/helper.c > +++ b/tools/virtiofsd/helper.c > @@ -151,6 +151,9 @@ void fuse_cmdline_help(void) > " -o cache= cache mode. could be one of \"auto, " > "always, none\"\n" > " default: auto\n" > + " -o chroot|no_chroot use container-friendly chroot instead\n" > + " of stronger mount namespace sandbox\n" > + " default: false\n" > " -o flock|no_flock enable/disable flock\n" > " default: no_flock\n" > " -o log_level= log level, default to \"info\"\n" > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > index 50a164a599..990c0a8a70 100644 > --- a/tools/virtiofsd/passthrough_ll.c > +++ b/tools/virtiofsd/passthrough_ll.c > @@ -139,6 +139,7 @@ enum { > > struct lo_data { > pthread_mutex_t mutex; > + int chroot; /* 1 - use chroot, 0 - use mount namespace */ > int debug; > int writeback; > int flock; > @@ -162,6 +163,8 @@ struct lo_data { > }; > > static const struct fuse_opt lo_opts[] = { > + { "chroot", offsetof(struct lo_data, chroot), 1 }, > + { "no_chroot", offsetof(struct lo_data, chroot), 0 }, > { "writeback", offsetof(struct lo_data, writeback), 1 }, > { "no_writeback", offsetof(struct lo_data, writeback), 0 }, > { "source=%s", offsetof(struct lo_data, source), 0 }, > @@ -2665,6 +2668,37 @@ static void setup_capabilities(char *modcaps_in) > pthread_mutex_unlock(&cap.mutex); > } > > +/* > + * Use chroot as a weaker sandbox for environment 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. > + * > + * It's still possible to escape the chroot via lo->proc_self_fd but that > + * requires gaining control of the process first. > + */ > + if (chroot(lo->source) != 0) { > + fuse_log(FUSE_LOG_ERR, "chroot(\"%s\"): %m\n", lo->source); > + exit(1); > + } I'm seeing suggestions that you should drop CAP_SYS_CHROOT after chroot'ing to stop an old escape (where you create another jail inside the jail and the kernel then lets you walk outside of the old one). Dave > + /* 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. > @@ -2672,8 +2706,13 @@ 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->chroot) { > + setup_chroot(lo); > + } else { > + setup_namespaces(lo, se); > + setup_mounts(lo->source); > + } > + > setup_seccomp(enable_syslog); > setup_capabilities(g_strdup(lo->modcaps)); > } > @@ -2820,6 +2859,7 @@ int main(int argc, char *argv[]) > struct fuse_session *se; > struct fuse_cmdline_opts opts; > struct lo_data lo = { > + .chroot = 0, > .debug = 0, > .writeback = 0, > .posix_lock = 1, > -- > 2.26.2 > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK