Re: [PATCH 2/2] virtiofsd: move to a new pid namespace

[Daniel P . Berrangé]

On Wed, Oct 16, 2019 at 05:01:57PM +0100, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> ---
>  contrib/virtiofsd/passthrough_ll.c | 95 ++++++++++++++++++++++--------
>  1 file changed, 72 insertions(+), 23 deletions(-)
> 
> diff --git a/contrib/virtiofsd/passthrough_ll.c 
> b/contrib/virtiofsd/passthrough_ll.c
> index c27ff7d800..b6ee9b2e90 100644
> --- a/contrib/virtiofsd/passthrough_ll.c
> +++ b/contrib/virtiofsd/passthrough_ll.c
> @@ -56,9 +56,12 @@
>  #include <sys/xattr.h>
>  #include <sys/mman.h>
>  #include <sys/socket.h>
> +#include <sys/types.h>
>  #include <sys/un.h>
> +#include <sys/wait.h>
>  #include <sys/capability.h>
>  
> +
>  #include "ireg.h"
>  #include <sys/mount.h>
>  #include <sys/resource.h>
> @@ -2749,6 +2752,72 @@ static void setup_net_namespace(void)
>       }
>  }
>  
> +/*
> + * Move to a new pid namespace to prevent access to other processes if this
> + * process is compromised.
> + */
> +static void setup_pid_namespace(void)
> +{
> +     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.
> +      */
> +     if (unshare(CLONE_NEWPID | CLONE_NEWNS) != 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);
> +
> +             if (WIFEXITED(wstatus)) {
> +                     exit(WEXITSTATUS(wstatus));
> +             }
> +
> +             exit(1);
> +     }

It might be useful to call prctl(PR_SET_PDEATHSIG) here, so that
if the parent process exits for any reason, the child will be killed
off too.

> +
> +     /*
> +      * 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);
> +     }
> +}

I feel like this is making things a bit misleading.

 setup_pid_namespace()

is now creating the mount namespace and pid namespace, and doing
some mount point config

 setup_mount_namespace()

is not creating the mount namespace, but is doing some more mount
point config.

And then there's setup_net_namespace() too.

I think there could be a  single

  setup_namespaces()

method that does the unshare(CLONE_NEWNS|CLONE_NEWNET|CLONE_NEWPID)
and forking the child.

And a setup_mounts()

method that does all the mount() calls.

> +
> +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);
> +     }
> +}
> +
>  /* This magic is based on lxc's lxc_pivot_root() */
>  static void setup_pivot_root(const char *source)
>  {
> @@ -2803,20 +2872,10 @@ static void setup_pivot_root(const char *source)
>  
>  /*
>   * Make the source directory our root so symlinks cannot escape and no other
> - * files are accessible.
> + * files are accessible.  Assumes unshare(CLONE_NEWNS) was already called.
>   */
>  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);
> @@ -2831,6 +2890,8 @@ static void setup_mount_namespace(const char *source)
>   */
>  static void setup_sandbox(struct lo_data *lo, bool enable_syslog)
>  {
> +     setup_pid_namespace();
> +     setup_proc_self_fd(lo);
>       setup_net_namespace();
>       setup_mount_namespace(lo->source);
>       setup_seccomp(enable_syslog);
> @@ -2860,15 +2921,6 @@ static void setup_root(struct lo_data *lo, struct 
> lo_inode *root)
>       g_atomic_int_set(&root->refcount, 2);
>  }
>  
> -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);
> -     }
> -}
> -
>  /* Raise the maximum number of open file descriptors to the system limit */
>  static void setup_nofile_rlimit(void)
>  {
> @@ -3110,9 +3162,6 @@ int main(int argc, char *argv[])
>               get_shared(&lo, &lo.root);
>       }
>  
> -     /* Must be after daemonize to get the right /proc/self/fd */
> -     setup_proc_self_fd(&lo);
> -
>       setup_sandbox(&lo, opts.syslog);
>  
>       setup_root(&lo, &lo.root);
> -- 
> 2.21.0
> 
> 

Regards,
Daniel
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