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From a7a87a751a9893830d031a957a751b7622b71fb2 Mon Sep 17 00:00:00 2001
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From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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Date: Mon, 27 Jan 2020 19:01:29 +0100
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Subject: [PATCH 058/116] virtiofsd: move to a new pid namespace
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
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Message-id: <20200127190227.40942-55-dgilbert@redhat.com>
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Patchwork-id: 93510
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O-Subject: [RHEL-AV-8.2 qemu-kvm PATCH 054/112] virtiofsd: move to a new pid namespace
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Bugzilla: 1694164
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RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
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From: Stefan Hajnoczi <stefanha@redhat.com>
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virtiofsd needs access to /proc/self/fd.  Let's move to a new pid
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namespace so that a compromised process cannot see another other
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processes running on the system.
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One wrinkle in this approach: unshare(CLONE_NEWPID) affects *child*
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processes and not the current process.  Therefore we need to fork the
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pid 1 process that will actually run virtiofsd and leave a parent in
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waitpid(2).  This is not the same thing as daemonization and parent
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processes should not notice a difference.
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
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Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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(cherry picked from commit 8e1d4ef231d8327be219f7aea7aa15d181375bbc)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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 tools/virtiofsd/passthrough_ll.c | 134 +++++++++++++++++++++++++--------------
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 1 file changed, 86 insertions(+), 48 deletions(-)
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diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
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index 27ab328..0947d14 100644
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--- a/tools/virtiofsd/passthrough_ll.c
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+++ b/tools/virtiofsd/passthrough_ll.c
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@@ -51,7 +51,10 @@
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 #include <string.h>
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 #include <sys/file.h>
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 #include <sys/mount.h>
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+#include <sys/prctl.h>
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 #include <sys/syscall.h>
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+#include <sys/types.h>
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+#include <sys/wait.h>
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 #include <sys/xattr.h>
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 #include <unistd.h>
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@@ -1945,24 +1948,95 @@ static void print_capabilities(void)
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 }
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 /*
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- * Called after our UNIX domain sockets have been created, now we can move to
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- * an empty network namespace to prevent TCP/IP and other network activity in
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- * case this process is compromised.
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+ * Move to a new mount, net, and pid namespaces to isolate this process.
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  */
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-static void setup_net_namespace(void)
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+static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
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 {
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-    if (unshare(CLONE_NEWNET) != 0) {
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-        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNET): %m\n");
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+    pid_t child;
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+
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+    /*
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+     * Create a new pid namespace for *child* processes.  We'll have to
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+     * fork in order to enter the new pid namespace.  A new mount namespace
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+     * is also needed so that we can remount /proc for the new pid
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+     * namespace.
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+     *
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+     * Our UNIX domain sockets have been created.  Now we can move to
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+     * an empty network namespace to prevent TCP/IP and other network
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+     * activity in case this process is compromised.
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+     */
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+    if (unshare(CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET) != 0) {
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+        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWPID | CLONE_NEWNS): %m\n");
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+        exit(1);
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+    }
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+
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+    child = fork();
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+    if (child < 0) {
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+        fuse_log(FUSE_LOG_ERR, "fork() failed: %m\n");
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+        exit(1);
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+    }
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+    if (child > 0) {
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+        pid_t waited;
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+        int wstatus;
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+
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+        /* The parent waits for the child */
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+        do {
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+            waited = waitpid(child, &wstatus, 0);
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+        } while (waited < 0 && errno == EINTR && !se->exited);
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+
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+        /* We were terminated by a signal, see fuse_signals.c */
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+        if (se->exited) {
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+            exit(0);
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+        }
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+
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+        if (WIFEXITED(wstatus)) {
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+            exit(WEXITSTATUS(wstatus));
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+        }
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+
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+        exit(1);
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+    }
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+
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+    /* Send us SIGTERM when the parent thread terminates, see prctl(2) */
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+    prctl(PR_SET_PDEATHSIG, SIGTERM);
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+
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+    /*
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+     * If the mounts have shared propagation then we want to opt out so our
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+     * mount changes don't affect the parent mount namespace.
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+     */
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+    if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
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+        fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_SLAVE): %m\n");
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+        exit(1);
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+    }
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+
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+    /* The child must remount /proc to use the new pid namespace */
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+    if (mount("proc", "/proc", "proc",
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+              MS_NODEV | MS_NOEXEC | MS_NOSUID | MS_RELATIME, NULL) < 0) {
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+        fuse_log(FUSE_LOG_ERR, "mount(/proc): %m\n");
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+        exit(1);
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+    }
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+
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+    /* Now we can get our /proc/self/fd directory file descriptor */
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+    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
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+    if (lo->proc_self_fd == -1) {
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+        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
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         exit(1);
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     }
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 }
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-/* This magic is based on lxc's lxc_pivot_root() */
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-static void setup_pivot_root(const char *source)
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+/*
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+ * Make the source directory our root so symlinks cannot escape and no other
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+ * files are accessible.  Assumes unshare(CLONE_NEWNS) was already called.
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+ */
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+static void setup_mounts(const char *source)
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 {
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     int oldroot;
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     int newroot;
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+    if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
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+        fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
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+        exit(1);
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+    }
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+
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+    /* This magic is based on lxc's lxc_pivot_root() */
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     oldroot = open("/", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
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     if (oldroot < 0) {
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         fuse_log(FUSE_LOG_ERR, "open(/): %m\n");
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@@ -2009,47 +2083,14 @@ static void setup_pivot_root(const char *source)
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     close(oldroot);
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 }
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-static void setup_proc_self_fd(struct lo_data *lo)
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-{
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-    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
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-    if (lo->proc_self_fd == -1) {
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-        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
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-        exit(1);
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-    }
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-}
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-
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-/*
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- * Make the source directory our root so symlinks cannot escape and no other
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- * files are accessible.
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- */
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-static void setup_mount_namespace(const char *source)
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-{
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-    if (unshare(CLONE_NEWNS) != 0) {
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-        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNS): %m\n");
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-        exit(1);
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-    }
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-
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-    if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
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-        fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_PRIVATE): %m\n");
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-        exit(1);
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-    }
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-
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-    if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
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-        fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
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-        exit(1);
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-    }
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-
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-    setup_pivot_root(source);
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-}
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-
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 /*
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  * Lock down this process to prevent access to other processes or files outside
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  * source directory.  This reduces the impact of arbitrary code execution bugs.
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  */
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-static void setup_sandbox(struct lo_data *lo)
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+static void setup_sandbox(struct lo_data *lo, struct fuse_session *se)
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 {
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-    setup_net_namespace();
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-    setup_mount_namespace(lo->source);
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+    setup_namespaces(lo, se);
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+    setup_mounts(lo->source);
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 }
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 int main(int argc, char *argv[])
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@@ -2173,10 +2214,7 @@ int main(int argc, char *argv[])
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     fuse_daemonize(opts.foreground);
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-    /* Must be after daemonize to get the right /proc/self/fd */
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-    setup_proc_self_fd(&lo);
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-
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-    setup_sandbox(&lo);
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+    setup_sandbox(&lo, se);
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     /* Block until ctrl+c or fusermount -u */
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     ret = virtio_loop(se);
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-- 
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1.8.3.1
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