[PATCH 1/2] libxl: fix stale fd event callback race

[Ian Jackson <ian.jackson@eu.citrix.com>]

Because there is not necessarily any lock held at the point the
application (eg, libvirt) calls libxl_osevent_occurred_timeout and
..._fd, in a multithreaded program those calls may be arbitrarily
delayed in relation to other activities within the program.

libxl therefore needs to be prepared to receive very old event
callbacks.  Arrange for this to be the case for fd callbacks.

This requires a new layer of indirection through a "hook nexus" struct
which can outlive the libxl__ev_foo.  Allocation and deallocation of
these nexi is mostly handled in the OSEVENT macros which wrap up
the application's callbacks.

Document the problem and the solution in a comment in libxl_event.c
just before the definition of struct libxl__osevent_hook_nexus.

There is still a race relating to libxl__osevent_occurred_timeout;
this will be addressed in the following patch.

Reported-by: Bamvor Jian Zhang <bjzhang@suse.com>
Cc: Bamvor Jian Zhang <bjzhang@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

--
v2:
  - Prepare for fixing timeout race too
  - Break out osevent_release_nexus()
  - nexusop argument to OSEVENT* macros
  - Clarify OSEVENT* nexusop hooks
  - osevent_ev_from_hook_nexus takes a libxl__osevent_hook_nexus*
---
 tools/libxl/libxl_event.c    |  184 +++++++++++++++++++++++++++++++++++------
 tools/libxl/libxl_internal.h |    8 ++-
 2 files changed, 163 insertions(+), 29 deletions(-)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index 72cb723..f1fe425 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -38,23 +38,131 @@
  * The application's registration hooks should be called ONLY via
  * these macros, with the ctx locked.  Likewise all the "occurred"
  * entrypoints from the application should assert(!in_hook);
+ *
+ * During the hook call - including while the arguments are being
+ * evaluated - ev->nexus is guaranteed to be valid and refer to the
+ * nexus which is being used for this event registration.  The
+ * arguments should specify ev->nexus for the for_libxl argument and
+ * ev->nexus->for_app_reg (or a pointer to it) for for_app_reg.
  */
-#define OSEVENT_HOOK_INTERN(retval, hookname, ...) do {                      \
-    if (CTX->osevent_hooks) {                                                \
-        CTX->osevent_in_hook++;                                              \
-        retval CTX->osevent_hooks->hookname(CTX->osevent_user, __VA_ARGS__); \
-        CTX->osevent_in_hook--;                                              \
-    }                                                                        \
+#define OSEVENT_HOOK_INTERN(retval, failedp, evkind, hookop, nexusop, ...) do { \
+    if (CTX->osevent_hooks) {                                           \
+        CTX->osevent_in_hook++;                                         \
+        libxl__osevent_hook_nexi *nexi = &CTX->hook_##evkind##_nexi_idle; \
+        osevent_hook_pre_##nexusop(gc, ev, nexi, &ev->nexus);            \
+        retval CTX->osevent_hooks->evkind##_##hookop                    \
+            (CTX->osevent_user, __VA_ARGS__);                           \
+        if ((failedp))                                                  \
+            osevent_hook_failed_##nexusop(gc, ev, nexi, &ev->nexus);     \
+        CTX->osevent_in_hook--;                                         \
+    }                                                                   \
 } while (0)
 
-#define OSEVENT_HOOK(hookname, ...) ({                                       \
-    int osevent_hook_rc = 0;                                                 \
-    OSEVENT_HOOK_INTERN(osevent_hook_rc = , hookname, __VA_ARGS__);          \
-    osevent_hook_rc;                                                         \
+#define OSEVENT_HOOK(evkind, hookop, nexusop, ...) ({                   \
+    int osevent_hook_rc = 0;                                    \
+    OSEVENT_HOOK_INTERN(osevent_hook_rc =, !!osevent_hook_rc,   \
+                        evkind, hookop, nexusop, __VA_ARGS__);          \
+    osevent_hook_rc;                                            \
 })
 
-#define OSEVENT_HOOK_VOID(hookname, ...) \
-    OSEVENT_HOOK_INTERN(/* void */, hookname, __VA_ARGS__)
+#define OSEVENT_HOOK_VOID(evkind, hookop, nexusop, ...)                         \
+    OSEVENT_HOOK_INTERN(/* void */, 0, evkind, hookop, nexusop, __VA_ARGS__)
+
+/*
+ * The application's calls to libxl_osevent_occurred_... may be
+ * indefinitely delayed with respect to the rest of the program (since
+ * they are not necessarily called with any lock held).  So the
+ * for_libxl value we receive may be (almost) arbitrarily old.  All we
+ * know is that it came from this ctx.
+ *
+ * Therefore we may not free the object referred to by any for_libxl
+ * value until we free the whole libxl_ctx.  And if we reuse it we
+ * must be able to tell when an old use turns up, and discard the
+ * stale event.
+ *
+ * Thus we cannot use the ev directly as the for_libxl value - we need
+ * a layer of indirection.
+ *
+ * We do this by keeping a pool of libxl__osevent_hook_nexus structs,
+ * and use pointers to them as for_libxl values.  In fact, there are
+ * two pools: one for fds and one for timeouts.  This ensures that we
+ * don't risk a type error when we upcast nexus->ev.  In each nexus
+ * the ev is either null or points to a valid libxl__ev_time or
+ * libxl__ev_fd, as applicable.
+ *
+ * We /do/ allow ourselves to reassociate an old nexus with a new ev
+ * as otherwise we would have to leak nexi.  (This reassociation
+ * might, of course, be an old ev being reused for a new purpose so
+ * simply comparing the ev pointer is not sufficient.)  Thus the
+ * libxl_osevent_occurred functions need to check that the condition
+ * allegedly signalled by this event actually exists.
+ *
+ * The nexi and the lists are all protected by the ctx lock.
+ */
+ 
+struct libxl__osevent_hook_nexus {
+    void *ev;
+    void *for_app_reg;
+    LIBXL_SLIST_ENTRY(libxl__osevent_hook_nexus) next;
+};
+
+static void *osevent_ev_from_hook_nexus(libxl_ctx *ctx,
+           libxl__osevent_hook_nexus *nexus /* pass  void *for_libxl */)
+{
+    return nexus->ev;
+}
+
+static void osevent_release_nexus(libxl__gc *gc,
+                                  libxl__osevent_hook_nexi *nexi_idle,
+                                  libxl__osevent_hook_nexus *nexus)
+{
+    nexus->ev = 0;
+    LIBXL_SLIST_INSERT_HEAD(nexi_idle, nexus, next);
+}
+
+/*----- OSEVENT* hook functions for nexusop "alloc" -----*/
+static void osevent_hook_pre_alloc(libxl__gc *gc, void *ev,
+                                   libxl__osevent_hook_nexi *nexi_idle,
+                                   libxl__osevent_hook_nexus **nexus_r)
+{
+    libxl__osevent_hook_nexus *nexus = LIBXL_SLIST_FIRST(nexi_idle);
+    if (nexus) {
+        LIBXL_SLIST_REMOVE_HEAD(nexi_idle, next);
+    } else {
+        nexus = libxl__zalloc(NOGC, sizeof(*nexus));
+    }
+    nexus->ev = ev;
+    *nexus_r = nexus;
+}
+static void osevent_hook_failed_alloc(libxl__gc *gc, void *ev,
+                                      libxl__osevent_hook_nexi *nexi_idle,
+                                      libxl__osevent_hook_nexus **nexus)
+{
+    osevent_release_nexus(gc, nexi_idle, *nexus);
+}
+
+/*----- OSEVENT* hook functions for nexusop "release" -----*/
+static void osevent_hook_pre_release(libxl__gc *gc, void *ev,
+                                     libxl__osevent_hook_nexi *nexi_idle,
+                                     libxl__osevent_hook_nexus **nexus)
+{
+    osevent_release_nexus(gc, nexi_idle, *nexus);
+}
+static void osevent_hook_failed_release(libxl__gc *gc, void *ev,
+                                        libxl__osevent_hook_nexi *nexi_idle,
+                                        libxl__osevent_hook_nexus **nexus)
+{
+    abort();
+}
+
+/*----- OSEVENT* hook functions for nexusop "noop" -----*/
+static void osevent_hook_pre_noop(libxl__gc *gc, void *ev,
+                                  libxl__osevent_hook_nexi *nexi_idle,
+                                  libxl__osevent_hook_nexus **nexus) { }
+static void osevent_hook_failed_noop(libxl__gc *gc, void *ev,
+                                     libxl__osevent_hook_nexi *nexi_idle,
+                                     libxl__osevent_hook_nexus **nexus) { }
+
 
 /*
  * fd events
@@ -72,7 +180,8 @@ int libxl__ev_fd_register(libxl__gc *gc, libxl__ev_fd *ev,
 
     DBG("ev_fd=%p register fd=%d events=%x", ev, fd, events);
 
-    rc = OSEVENT_HOOK(fd_register, fd, &ev->for_app_reg, events, ev);
+    rc = OSEVENT_HOOK(fd,register, alloc, fd, &ev->nexus->for_app_reg,
+                      events, ev->nexus);
     if (rc) goto out;
 
     ev->fd = fd;
@@ -97,7 +206,7 @@ int libxl__ev_fd_modify(libxl__gc *gc, libxl__ev_fd *ev, short events)
 
     DBG("ev_fd=%p modify fd=%d events=%x", ev, ev->fd, events);
 
-    rc = OSEVENT_HOOK(fd_modify, ev->fd, &ev->for_app_reg, events);
+    rc = OSEVENT_HOOK(fd,modify, noop, ev->fd, &ev->nexus->for_app_reg, events);
     if (rc) goto out;
 
     ev->events = events;
@@ -119,7 +228,7 @@ void libxl__ev_fd_deregister(libxl__gc *gc, libxl__ev_fd *ev)
 
     DBG("ev_fd=%p deregister fd=%d", ev, ev->fd);
 
-    OSEVENT_HOOK_VOID(fd_deregister, ev->fd, ev->for_app_reg);
+    OSEVENT_HOOK_VOID(fd,deregister, release, ev->fd, ev->nexus->for_app_reg);
     LIBXL_LIST_REMOVE(ev, entry);
     ev->fd = -1;
 
@@ -171,7 +280,8 @@ static int time_register_finite(libxl__gc *gc, libxl__ev_time *ev,
 {
     int rc;
 
-    rc = OSEVENT_HOOK(timeout_register, &ev->for_app_reg, absolute, ev);
+    rc = OSEVENT_HOOK(timeout,register, alloc, &ev->nexus->for_app_reg,
+                      absolute, ev->nexus);
     if (rc) return rc;
 
     ev->infinite = 0;
@@ -184,7 +294,7 @@ static int time_register_finite(libxl__gc *gc, libxl__ev_time *ev,
 static void time_deregister(libxl__gc *gc, libxl__ev_time *ev)
 {
     if (!ev->infinite) {
-        OSEVENT_HOOK_VOID(timeout_deregister, ev->for_app_reg);
+        OSEVENT_HOOK_VOID(timeout,deregister, release, ev->nexus->for_app_reg);
         LIBXL_TAILQ_REMOVE(&CTX->etimes, ev, entry);
     }
 }
@@ -270,7 +380,8 @@ int libxl__ev_time_modify_abs(libxl__gc *gc, libxl__ev_time *ev,
         rc = time_register_finite(gc, ev, absolute);
         if (rc) goto out;
     } else {
-        rc = OSEVENT_HOOK(timeout_modify, &ev->for_app_reg, absolute);
+        rc = OSEVENT_HOOK(timeout,modify, noop,
+                          &ev->nexus->for_app_reg, absolute);
         if (rc) goto out;
 
         LIBXL_TAILQ_REMOVE(&CTX->etimes, ev, entry);
@@ -1010,35 +1121,54 @@ void libxl_osevent_register_hooks(libxl_ctx *ctx,
 
 
 void libxl_osevent_occurred_fd(libxl_ctx *ctx, void *for_libxl,
-                               int fd, short events, short revents)
+                               int fd, short events_ign, short revents_ign)
 {
-    libxl__ev_fd *ev = for_libxl;
-
     EGC_INIT(ctx);
     CTX_LOCK;
     assert(!CTX->osevent_in_hook);
 
-    assert(fd == ev->fd);
-    revents &= ev->events;
-    if (revents)
-        ev->func(egc, ev, fd, ev->events, revents);
+    libxl__ev_fd *ev = osevent_ev_from_hook_nexus(ctx, for_libxl);
+    if (!ev) goto out;
+    if (ev->fd != fd) goto out;
 
+    struct pollfd check;
+    for (;;) {
+        check.fd = fd;
+        check.events = ev->events;
+        int r = poll(&check, 1, 0);
+        if (!r)
+            goto out;
+        if (r==1)
+            break;
+        assert(r<0);
+        if (errno != EINTR) {
+            LIBXL__EVENT_DISASTER(egc, "failed poll to check for fd", errno, 0);
+            goto out;
+        }
+    }
+
+    if (check.revents)
+        ev->func(egc, ev, fd, ev->events, check.revents);
+
+ out:
     CTX_UNLOCK;
     EGC_FREE;
 }
 
 void libxl_osevent_occurred_timeout(libxl_ctx *ctx, void *for_libxl)
 {
-    libxl__ev_time *ev = for_libxl;
-
     EGC_INIT(ctx);
     CTX_LOCK;
     assert(!CTX->osevent_in_hook);
 
+    libxl__ev_time *ev = osevent_ev_from_hook_nexus(ctx, for_libxl);
+    if (!ev) goto out;
     assert(!ev->infinite);
+
     LIBXL_TAILQ_REMOVE(&CTX->etimes, ev, entry);
     ev->func(egc, ev, &ev->abs);
 
+ out:
     CTX_UNLOCK;
     EGC_FREE;
 }
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index cba3616..6484bcb 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -136,6 +136,8 @@ typedef struct libxl__gc libxl__gc;
 typedef struct libxl__egc libxl__egc;
 typedef struct libxl__ao libxl__ao;
 typedef struct libxl__aop_occurred libxl__aop_occurred;
+typedef struct libxl__osevent_hook_nexus libxl__osevent_hook_nexus;
+typedef struct libxl__osevent_hook_nexi libxl__osevent_hook_nexi;
 
 _hidden void libxl__alloc_failed(libxl_ctx *, const char *func,
                          size_t nmemb, size_t size) __attribute__((noreturn));
@@ -163,7 +165,7 @@ struct libxl__ev_fd {
     libxl__ev_fd_callback *func;
     /* remainder is private for libxl__ev_fd... */
     LIBXL_LIST_ENTRY(libxl__ev_fd) entry;
-    void *for_app_reg;
+    libxl__osevent_hook_nexus *nexus;
 };
 
 
@@ -178,7 +180,7 @@ struct libxl__ev_time {
     int infinite; /* not registered in list or with app if infinite */
     LIBXL_TAILQ_ENTRY(libxl__ev_time) entry;
     struct timeval abs;
-    void *for_app_reg;
+    libxl__osevent_hook_nexus *nexus;
 };
 
 typedef struct libxl__ev_xswatch libxl__ev_xswatch;
@@ -329,6 +331,8 @@ struct libxl__ctx {
     libxl__poller poller_app; /* libxl_osevent_beforepoll and _afterpoll */
     LIBXL_LIST_HEAD(, libxl__poller) pollers_event, pollers_idle;
 
+    LIBXL_SLIST_HEAD(libxl__osevent_hook_nexi, libxl__osevent_hook_nexus)
+        hook_fd_nexi_idle, hook_timeout_nexi_idle;
     LIBXL_LIST_HEAD(, libxl__ev_fd) efds;
     LIBXL_TAILQ_HEAD(, libxl__ev_time) etimes;
 
-- 
1.7.2.5