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Fri, 7 Aug 2020 15:36:30 +0000 (UTC) Date: Fri, 7 Aug 2020 16:36:28 +0100 From: "Dr. David Alan Gilbert" To: Stefan Hajnoczi Subject: Re: [PATCH v2 2/3] virtiofsd: add container-friendly -o sandbox=chroot option Message-ID: <20200807153628.GG2780@work-vm> References: <20200727190223.422280-1-stefanha@redhat.com> <20200727190223.422280-3-stefanha@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200727190223.422280-3-stefanha@redhat.com> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@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=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 = 2020/08/07 02:21:37 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_H3=-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: vromanso@redhat.com, Daniel Walsh , qemu-devel@nongnu.org, rmohr@redhat.com, virtio-fs@redhat.com, mpatel@redhat.com, vgoyal@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 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 Is there anyone with a bit more FS semantics expertise who could check this; I already surprised myself a few times reading about chroot escapes, so I'd appreciate a 2nd pair of eyes. Dave > --- > 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 3105b6c23a..91dcb23664 100644 > --- a/tools/virtiofsd/helper.c > +++ b/tools/virtiofsd/helper.c > @@ -168,6 +168,14 @@ 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 50a164a599..a7894c3e7c 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; > @@ -162,6 +168,12 @@ 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 }, > @@ -2665,6 +2677,41 @@ 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. > @@ -2672,8 +2719,13 @@ 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)); > } > @@ -2820,6 +2872,7 @@ 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 = 1, > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst > index 824e713491..40629f95ae 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 > ------- > @@ -72,6 +83,13 @@ 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 7 Aug 2020 16:36:28 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20200807153628.GG2780@work-vm> References: <20200727190223.422280-1-stefanha@redhat.com> <20200727190223.422280-3-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200727190223.422280-3-stefanha@redhat.com> Subject: Re: [Virtio-fs] [PATCH v2 2/3] virtiofsd: add container-friendly -o sandbox=chroot option List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: vromanso@redhat.com, qemu-devel@nongnu.org, rmohr@redhat.com, virtio-fs@redhat.com, mpatel@redhat.com, vgoyal@redhat.com * 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 Is there anyone with a bit more FS semantics expertise who could check this; I already surprised myself a few times reading about chroot escapes, so I'd appreciate a 2nd pair of eyes. Dave > --- > 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 3105b6c23a..91dcb23664 100644 > --- a/tools/virtiofsd/helper.c > +++ b/tools/virtiofsd/helper.c > @@ -168,6 +168,14 @@ 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 50a164a599..a7894c3e7c 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; > @@ -162,6 +168,12 @@ 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 }, > @@ -2665,6 +2677,41 @@ 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. > @@ -2672,8 +2719,13 @@ 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)); > } > @@ -2820,6 +2872,7 @@ 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 = 1, > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst > index 824e713491..40629f95ae 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 > ------- > @@ -72,6 +83,13 @@ 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