From: Stefan Hajnoczi <stefanha@redhat.com>

Date: Mon, 27 Jan 2020 19:01:29 +0000

Subject: [PATCH] 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>

(cherry picked from commit 8e1d4ef231d8327be219f7aea7aa15d181375bbc)

---

 tools/virtiofsd/passthrough_ll.c | 134 ++++++++++++++++++++-----------

 1 file changed, 86 insertions(+), 48 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c

index 27ab328722..0947d14e5b 100644

--- a/tools/virtiofsd/passthrough_ll.c

+++ b/tools/virtiofsd/passthrough_ll.c

@@ -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);