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[PULL 03/16] virtiofsd: add container-friendly -o sandbox=chroot option
From: |
Dr. David Alan Gilbert (git) |
Subject: |
[PULL 03/16] virtiofsd: add container-friendly -o sandbox=chroot option |
Date: |
Mon, 26 Oct 2020 18:43:18 +0000 |
From: Stefan Hajnoczi <stefanha@redhat.com>
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 <stefanha@redhat.com>
Message-Id: <20201008085534.16070-1-stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
docs/tools/virtiofsd.rst | 32 ++++++++++++++----
tools/virtiofsd/helper.c | 8 +++++
tools/virtiofsd/passthrough_ll.c | 57 ++++++++++++++++++++++++++++++--
3 files changed, 88 insertions(+), 9 deletions(-)
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.
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=<number> 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 13fdb12367..f03b1f9a69 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,
--
2.28.0
- [PULL 00/16] virtiofs queue, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 01/16] configure: add option for virtiofsd, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 02/16] virtiofsd: passthrough_ll: set FUSE_LOG_INFO as default log_level, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 03/16] virtiofsd: add container-friendly -o sandbox=chroot option,
Dr. David Alan Gilbert (git) <=
- [PULL 04/16] tools/virtiofsd: xattr name mappings: Add option, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 05/16] tools/virtiofsd: xattr name mappings: Map client xattr names, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 07/16] tools/virtiofsd: xattr name mapping examples, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 06/16] tools/virtiofsd: xattr name mappings: Map server xattr names, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 08/16] tools/virtiofsd: xattr name mappings: Simple 'map', Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 09/16] linux/fuse.h: Pull in from Linux, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 11/16] virtiofsd: Add attr_flags to fuse_entry_param, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 10/16] virtiofsd: Announce FUSE_ATTR_FLAGS, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 13/16] virtiofsd: Store every lo_inode's parent_dev, Dr. David Alan Gilbert (git), 2020/10/26
- [PULL 12/16] virtiofsd: Add fuse_reply_attr_with_flags(), Dr. David Alan Gilbert (git), 2020/10/26