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virtiofsd: move to a new pid namespace

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virtiofsd needs access to /proc/self/fd.  Let's move to a new pid
namespace so that a compromised process cannot see another other
processes running on the system.

One wrinkle in this approach: unshare(CLONE_NEWPID) affects *child*
processes and not the current process.  Therefore we need to fork the
pid 1 process that will actually run virtiofsd and leave a parent in
waitpid(2).  This is not the same thing as daemonization and parent
processes should not notice a difference.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2019-10-16 17:01:57 +01:00 committed by Dr. David Alan Gilbert
parent d74830d12a
commit 8e1d4ef231
1 changed files with 86 additions and 48 deletions
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134
tools/virtiofsd/passthrough_ll.c
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@ -51,7 +51,10 @@
#include <string.h>
#include <sys/file.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/xattr.h>
#include <unistd.h>
@ -1945,24 +1948,95 @@ static void print_capabilities(void)
}
/*
* Called after our UNIX domain sockets have been created, now we can move to
* an empty network namespace to prevent TCP/IP and other network activity in
* case this process is compromised.
* Move to a new mount, net, and pid namespaces to isolate this process.
*/
static void setup_net_namespace(void)
static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
{
if (unshare(CLONE_NEWNET) != 0) {
fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNET): %m\n");
pid_t child;
/*
* Create a new pid namespace for *child* processes. We'll have to
* fork in order to enter the new pid namespace. A new mount namespace
* is also needed so that we can remount /proc for the new pid
* namespace.
*
* Our UNIX domain sockets have been created. Now we can move to
* an empty network namespace to prevent TCP/IP and other network
* activity in case this process is compromised.
*/
if (unshare(CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET) != 0) {
fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWPID | CLONE_NEWNS): %m\n");
exit(1);
}
child = fork();
if (child < 0) {
fuse_log(FUSE_LOG_ERR, "fork() failed: %m\n");
exit(1);
}
if (child > 0) {
pid_t waited;
int wstatus;
/* The parent waits for the child */
do {
waited = waitpid(child, &wstatus, 0);
} while (waited < 0 && errno == EINTR && !se->exited);
/* We were terminated by a signal, see fuse_signals.c */
if (se->exited) {
exit(0);
}
if (WIFEXITED(wstatus)) {
exit(WEXITSTATUS(wstatus));
}
exit(1);
}
/* Send us SIGTERM when the parent thread terminates, see prctl(2) */
prctl(PR_SET_PDEATHSIG, SIGTERM);
/*
* If the mounts have shared propagation then we want to opt out so our
* mount changes don't affect the parent mount namespace.
*/
if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_SLAVE): %m\n");
exit(1);
}
/* The child must remount /proc to use the new pid namespace */
if (mount("proc", "/proc", "proc",
MS_NODEV | MS_NOEXEC | MS_NOSUID | MS_RELATIME, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(/proc): %m\n");
exit(1);
}
/* Now we can get our /proc/self/fd directory file descriptor */
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);
}
}
/* This magic is based on lxc's lxc_pivot_root() */
static void setup_pivot_root(const char *source)
/*
* Make the source directory our root so symlinks cannot escape and no other
* files are accessible. Assumes unshare(CLONE_NEWNS) was already called.
*/
static void setup_mounts(const char *source)
{
int oldroot;
int newroot;
if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
exit(1);
}
/* This magic is based on lxc's lxc_pivot_root() */
oldroot = open("/", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
if (oldroot < 0) {
fuse_log(FUSE_LOG_ERR, "open(/): %m\n");
@ -2009,47 +2083,14 @@ static void setup_pivot_root(const char *source)
close(oldroot);
}
static void setup_proc_self_fd(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 source directory our root so symlinks cannot escape and no other
* files are accessible.
*/
static void setup_mount_namespace(const char *source)
{
if (unshare(CLONE_NEWNS) != 0) {
fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNS): %m\n");
exit(1);
}
if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_PRIVATE): %m\n");
exit(1);
}
if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
exit(1);
}
setup_pivot_root(source);
}
/*
* Lock down this process to prevent access to other processes or files outside
* source directory. This reduces the impact of arbitrary code execution bugs.
*/
static void setup_sandbox(struct lo_data *lo)
static void setup_sandbox(struct lo_data *lo, struct fuse_session *se)
{
setup_net_namespace();
setup_mount_namespace(lo->source);
setup_namespaces(lo, se);
setup_mounts(lo->source);
}
int main(int argc, char *argv[])
@ -2173,10 +2214,7 @@ int main(int argc, char *argv[])
fuse_daemonize(opts.foreground);
/* Must be after daemonize to get the right /proc/self/fd */
setup_proc_self_fd(&lo);
setup_sandbox(&lo);
setup_sandbox(&lo, se);
/* Block until ctrl+c or fusermount -u */
ret = virtio_loop(se);