qemu-ga: use key-value store to avoid recycling fd handles after restart

Hosts hold on to handles provided by guest-file-open for periods that can
span beyond the life of the qemu-ga process that issued them. Since these
are issued starting from 0 on every restart, we run the risk of issuing
duplicate handles after restarts/reboots.

As a result, users with a stale copy of these handles may end up
reading/writing corrupted data due to their existing handles effectively
being re-assigned to an unexpected file or offset.

We unfortunately do not issue handles as strings, but as integers, so a
solution such as using UUIDs can't be implemented without introducing a
new interface.

As a workaround, we fix this by implementing a persistent key-value store
that will be used to track the value of the last handle that was issued
across restarts/reboots to avoid issuing duplicates.

The store is automatically written to the same directory we currently
set via --statedir to track fsfreeze state, and so should be applicable
for stable releases where this flag is supported.

A follow-up can use this same store for handling fsfreeze state, but
that change is cosmetic and left out for now.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org

* fixed guest_file_handle_add() return value from uint64_t to int64_t

1 files changed, 184 insertions, 0 deletions

diff --git a/qga/main.c b/qga/main.c
index ad75171..99346e1 100644
--- a/qga/main.c
+++ b/qga/main.c
@@ -15,6 +15,7 @@
 #include <stdbool.h>
 #include <glib.h>
 #include <getopt.h>
+#include <glib/gstdio.h>
 #ifndef _WIN32
 #include <syslog.h>
 #include <sys/wait.h>
@@ -30,6 +31,7 @@
 #include "qapi/qmp/qerror.h"
 #include "qapi/qmp/dispatch.h"
 #include "qga/channel.h"
+#include "qemu/bswap.h"
 #ifdef _WIN32
 #include "qga/service-win32.h"
 #include <windows.h>
@@ -53,6 +55,11 @@
 #endif
 #define QGA_SENTINEL_BYTE 0xFF
 
+typedef struct GAPersistentState {
+#define QGA_PSTATE_DEFAULT_FD_COUNTER 1000
+    int64_t fd_counter;
+} GAPersistentState;
+
 struct GAState {
     JSONMessageParser parser;
     GMainLoop *main_loop;
@@ -76,6 +83,8 @@ struct GAState {
 #ifdef CONFIG_FSFREEZE
     const char *fsfreeze_hook;
 #endif
+    const gchar *pstate_filepath;
+    GAPersistentState pstate;
 };
 
 struct GAState *ga_state;
@@ -725,6 +734,171 @@ VOID WINAPI service_main(DWORD argc, TCHAR *argv[])
 }
 #endif
 
+static void set_persistent_state_defaults(GAPersistentState *pstate)
+{
+    g_assert(pstate);
+    pstate->fd_counter = QGA_PSTATE_DEFAULT_FD_COUNTER;
+}
+
+static void persistent_state_from_keyfile(GAPersistentState *pstate,
+                                          GKeyFile *keyfile)
+{
+    g_assert(pstate);
+    g_assert(keyfile);
+    /* if any fields are missing, either because the file was tampered with
+     * by agents of chaos, or because the field wasn't present at the time the
+     * file was created, the best we can ever do is start over with the default
+     * values. so load them now, and ignore any errors in accessing key-value
+     * pairs
+     */
+    set_persistent_state_defaults(pstate);
+
+    if (g_key_file_has_key(keyfile, "global", "fd_counter", NULL)) {
+        pstate->fd_counter =
+            g_key_file_get_int64(keyfile, "global", "fd_counter", NULL);
+    }
+}
+
+static void persistent_state_to_keyfile(const GAPersistentState *pstate,
+                                        GKeyFile *keyfile)
+{
+    g_assert(pstate);
+    g_assert(keyfile);
+
+    g_key_file_set_int64(keyfile, "global", "fd_counter", pstate->fd_counter);
+}
+
+static gboolean write_persistent_state(const GAPersistentState *pstate,
+                                       const gchar *path)
+{
+    GKeyFile *keyfile = g_key_file_new();
+    GError *gerr = NULL;
+    gboolean ret = true;
+    gchar *data = NULL;
+    gsize data_len;
+
+    g_assert(pstate);
+
+    persistent_state_to_keyfile(pstate, keyfile);
+    data = g_key_file_to_data(keyfile, &data_len, &gerr);
+    if (gerr) {
+        g_critical("failed to convert persistent state to string: %s",
+                   gerr->message);
+        ret = false;
+        goto out;
+    }
+
+    g_file_set_contents(path, data, data_len, &gerr);
+    if (gerr) {
+        g_critical("failed to write persistent state to %s: %s",
+                    path, gerr->message);
+        ret = false;
+        goto out;
+    }
+
+out:
+    if (gerr) {
+        g_error_free(gerr);
+    }
+    if (keyfile) {
+        g_key_file_free(keyfile);
+    }
+    g_free(data);
+    return ret;
+}
+
+static gboolean read_persistent_state(GAPersistentState *pstate,
+                                      const gchar *path, gboolean frozen)
+{
+    GKeyFile *keyfile = NULL;
+    GError *gerr = NULL;
+    struct stat st;
+    gboolean ret = true;
+
+    g_assert(pstate);
+
+    if (stat(path, &st) == -1) {
+        /* it's okay if state file doesn't exist, but any other error
+         * indicates a permissions issue or some other misconfiguration
+         * that we likely won't be able to recover from.
+         */
+        if (errno != ENOENT) {
+            g_critical("unable to access state file at path %s: %s",
+                       path, strerror(errno));
+            ret = false;
+            goto out;
+        }
+
+        /* file doesn't exist. initialize state to default values and
+         * attempt to save now. (we could wait till later when we have
+         * modified state we need to commit, but if there's a problem,
+         * such as a missing parent directory, we want to catch it now)
+         *
+         * there is a potential scenario where someone either managed to
+         * update the agent from a version that didn't use a key store
+         * while qemu-ga thought the filesystem was frozen, or
+         * deleted the key store prior to issuing a fsfreeze, prior
+         * to restarting the agent. in this case we go ahead and defer
+         * initial creation till we actually have modified state to
+         * write, otherwise fail to recover from freeze.
+         */
+        set_persistent_state_defaults(pstate);
+        if (!frozen) {
+            ret = write_persistent_state(pstate, path);
+            if (!ret) {
+                g_critical("unable to create state file at path %s", path);
+                ret = false;
+                goto out;
+            }
+        }
+        ret = true;
+        goto out;
+    }
+
+    keyfile = g_key_file_new();
+    g_key_file_load_from_file(keyfile, path, 0, &gerr);
+    if (gerr) {
+        g_critical("error loading persistent state from path: %s, %s",
+                   path, gerr->message);
+        ret = false;
+        goto out;
+    }
+
+    persistent_state_from_keyfile(pstate, keyfile);
+
+out:
+    if (keyfile) {
+        g_key_file_free(keyfile);
+    }
+    if (gerr) {
+        g_error_free(gerr);
+    }
+
+    return ret;
+}
+
+int64_t ga_get_fd_handle(GAState *s, Error **errp)
+{
+    int64_t handle;
+
+    g_assert(s->pstate_filepath);
+    /* we blacklist commands and avoid operations that potentially require
+     * writing to disk when we're in a frozen state. this includes opening
+     * new files, so we should never get here in that situation
+     */
+    g_assert(!ga_is_frozen(s));
+
+    handle = s->pstate.fd_counter++;
+    if (s->pstate.fd_counter < 0) {
+        s->pstate.fd_counter = 0;
+    }
+    if (!write_persistent_state(&s->pstate, s->pstate_filepath)) {
+        error_setg(errp, "failed to commit persistent state to disk");
+    }
+
+    return handle;
+}
+
 int main(int argc, char **argv)
 {
     const char *sopt = "hVvdm:p:l:f:F::b:s:t:";
@@ -854,7 +1028,9 @@ int main(int argc, char **argv)
     ga_enable_logging(s);
     s->state_filepath_isfrozen = g_strdup_printf("%s/qga.state.isfrozen",
                                                  state_dir);
+    s->pstate_filepath = g_strdup_printf("%s/qga.state", state_dir);
     s->frozen = false;
+
 #ifndef _WIN32
     /* check if a previous instance of qemu-ga exited with filesystems' state
      * marked as frozen. this could be a stale value (a non-qemu-ga process
@@ -911,6 +1087,14 @@ int main(int argc, char **argv)
         }
     }
 
+    /* load persistent state from disk */
+    if (!read_persistent_state(&s->pstate,
+                               s->pstate_filepath,
+                               ga_is_frozen(s))) {
+        g_critical("failed to load persistent state");
+        goto out_bad;
+    }
+
     if (blacklist) {
         s->blacklist = blacklist;
         do {