1 files changed, 158 insertions, 17 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cc8a9b7d6064..ed38801b57a7 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -62,7 +62,14 @@
  * This mutex is acquired by ep_free() during the epoll file
  * cleanup path and it is also acquired by eventpoll_release_file()
  * if a file has been pushed inside an epoll set and it is then
- * close()d without a previous call toepoll_ctl(EPOLL_CTL_DEL).
+ * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
+ * It is also acquired when inserting an epoll fd onto another epoll
+ * fd. We do this so that we walk the epoll tree and ensure that this
+ * insertion does not create a cycle of epoll file descriptors, which
+ * could lead to deadlock. We need a global mutex to prevent two
+ * simultaneous inserts (A into B and B into A) from racing and
+ * constructing a cycle without either insert observing that it is
+ * going to.
  * It is possible to drop the "ep->mtx" and to use the global
  * mutex "epmutex" (together with "ep->lock") to have it working,
  * but having "ep->mtx" will make the interface more scalable.
@@ -145,11 +152,11 @@ struct epitem {
 
 /*
  * This structure is stored inside the "private_data" member of the file
- * structure and rapresent the main data sructure for the eventpoll
+ * structure and represents the main data structure for the eventpoll
  * interface.
  */
 struct eventpoll {
-        /* Protect the this structure access */
+        /* Protect the access to this structure */
         spinlock_t lock;
 
         /*
@@ -224,6 +231,9 @@ static long max_user_watches __read_mostly;
  */
 static DEFINE_MUTEX(epmutex);
 
+/* Used to check for epoll file descriptor inclusion loops */
+static struct nested_calls poll_loop_ncalls;
+
 /* Used for safe wake up implementation */
 static struct nested_calls poll_safewake_ncalls;
 
@@ -306,6 +316,19 @@ static void ep_nested_calls_init(struct nested_calls *ncalls)
 }
 
 /**
+ * ep_events_available - Checks if ready events might be available.
+ *
+ * @ep: Pointer to the eventpoll context.
+ *
+ * Returns: Returns a value different than zero if ready events are available,
+ *          or zero otherwise.
+ */
+static inline int ep_events_available(struct eventpoll *ep)
+{
+        return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
+}
+
+/**
  * ep_call_nested - Perform a bound (possibly) nested call, by checking
  *                  that the recursion limit is not exceeded, and that
  *                  the same nested call (by the meaning of same cookie) is
@@ -783,7 +806,7 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
 
 /*
  * This is the callback that is passed to the wait queue wakeup
- * machanism. It is called by the stored file descriptors when they
+ * mechanism. It is called by the stored file descriptors when they
  * have events to report.
  */
 static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
@@ -814,9 +837,9 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
                 goto out_unlock;
 
         /*
-         * If we are trasfering events to userspace, we can hold no locks
+         * If we are transferring events to userspace, we can hold no locks
          * (because we're accessing user memory, and because of linux f_op->poll()
-         * semantics). All the events that happens during that period of time are
+         * semantics). All the events that happen during that period of time are
          * chained in ep->ovflist and requeued later on.
          */
         if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
@@ -1114,31 +1137,63 @@ static int ep_send_events(struct eventpoll *ep,
         return ep_scan_ready_list(ep, ep_send_events_proc, &esed);
 }
 
+static inline struct timespec ep_set_mstimeout(long ms)
+{
+        struct timespec now, ts = {
+                .tv_sec = ms / MSEC_PER_SEC,
+                .tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
+        };
+
+        ktime_get_ts(&now);
+        return timespec_add_safe(now, ts);
+}
+
+/**
+ * ep_poll - Retrieves ready events, and delivers them to the caller supplied
+ *           event buffer.
+ *
+ * @ep: Pointer to the eventpoll context.
+ * @events: Pointer to the userspace buffer where the ready events should be
+ *          stored.
+ * @maxevents: Size (in terms of number of events) of the caller event buffer.
+ * @timeout: Maximum timeout for the ready events fetch operation, in
+ *           milliseconds. If the @timeout is zero, the function will not block,
+ *           while if the @timeout is less than zero, the function will block
+ *           until at least one event has been retrieved (or an error
+ *           occurred).
+ *
+ * Returns: Returns the number of ready events which have been fetched, or an
+ *          error code, in case of error.
+ */
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
                    int maxevents, long timeout)
 {
-        int res, eavail, timed_out = 0;
+        int res = 0, eavail, timed_out = 0;
         unsigned long flags;
-        long slack;
+        long slack = 0;
         wait_queue_t wait;
-        struct timespec end_time;
         ktime_t expires, *to = NULL;
 
         if (timeout > 0) {
-                ktime_get_ts(&end_time);
-                timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
+                struct timespec end_time = ep_set_mstimeout(timeout);
+
                 slack = select_estimate_accuracy(&end_time);
                 to = &expires;
                 *to = timespec_to_ktime(end_time);
         } else if (timeout == 0) {
+                /*
+                 * Avoid the unnecessary trip to the wait queue loop, if the
+                 * caller specified a non blocking operation.
+                 */
                 timed_out = 1;
+                spin_lock_irqsave(&ep->lock, flags);
+                goto check_events;
         }
 
-retry:
+fetch_events:
         spin_lock_irqsave(&ep->lock, flags);
 
-        res = 0;
-        if (list_empty(&ep->rdllist)) {
+        if (!ep_events_available(ep)) {
                 /*
                  * We don't have any available event to return to the caller.
                  * We need to sleep here, and we will be wake up by
@@ -1154,7 +1209,7 @@ retry:
                          * to TASK_INTERRUPTIBLE before doing the checks.
                          */
                         set_current_state(TASK_INTERRUPTIBLE);
-                        if (!list_empty(&ep->rdllist) || timed_out)
+                        if (ep_events_available(ep) || timed_out)
                                 break;
                         if (signal_pending(current)) {
                                 res = -EINTR;
@@ -1171,8 +1226,9 @@ retry:
 
                 set_current_state(TASK_RUNNING);
         }
+check_events:
         /* Is it worth to try to dig for events ? */
-        eavail = !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
+        eavail = ep_events_available(ep);
 
         spin_unlock_irqrestore(&ep->lock, flags);
 
@@ -1183,11 +1239,67 @@ retry:
          */
         if (!res && eavail &&
             !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
-                goto retry;
+                goto fetch_events;
 
         return res;
 }
 
+/**
+ * ep_loop_check_proc - Callback function to be passed to the @ep_call_nested()
+ *                      API, to verify that adding an epoll file inside another
+ *                      epoll structure, does not violate the constraints, in
+ *                      terms of closed loops, or too deep chains (which can
+ *                      result in excessive stack usage).
+ *
+ * @priv: Pointer to the epoll file to be currently checked.
+ * @cookie: Original cookie for this call. This is the top-of-the-chain epoll
+ *          data structure pointer.
+ * @call_nests: Current dept of the @ep_call_nested() call stack.
+ *
+ * Returns: Returns zero if adding the epoll @file inside current epoll
+ *          structure @ep does not violate the constraints, or -1 otherwise.
+ */
+static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
+{
+        int error = 0;
+        struct file *file = priv;
+        struct eventpoll *ep = file->private_data;
+        struct rb_node *rbp;
+        struct epitem *epi;
+
+        mutex_lock(&ep->mtx);
+        for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+                epi = rb_entry(rbp, struct epitem, rbn);
+                if (unlikely(is_file_epoll(epi->ffd.file))) {
+                        error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+                                               ep_loop_check_proc, epi->ffd.file,
+                                               epi->ffd.file->private_data, current);
+                        if (error != 0)
+                                break;
+                }
+        }
+        mutex_unlock(&ep->mtx);
+
+        return error;
+}
+
+/**
+ * ep_loop_check - Performs a check to verify that adding an epoll file (@file)
+ *                 another epoll file (represented by @ep) does not create
+ *                 closed loops or too deep chains.
+ *
+ * @ep: Pointer to the epoll private data structure.
+ * @file: Pointer to the epoll file to be checked.
+ *
+ * Returns: Returns zero if adding the epoll @file inside current epoll
+ *          structure @ep does not violate the constraints, or -1 otherwise.
+ */
+static int ep_loop_check(struct eventpoll *ep, struct file *file)
+{
+        return ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+                              ep_loop_check_proc, file, ep, current);
+}
+
 /*
  * Open an eventpoll file descriptor.
  */
@@ -1236,6 +1348,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
                 struct epoll_event __user *, event)
 {
         int error;
+        int did_lock_epmutex = 0;
         struct file *file, *tfile;
         struct eventpoll *ep;
         struct epitem *epi;
@@ -1277,6 +1390,25 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
          */
         ep = file->private_data;
 
+        /*
+         * When we insert an epoll file descriptor, inside another epoll file
+         * descriptor, there is the change of creating closed loops, which are
+         * better be handled here, than in more critical paths.
+         *
+         * We hold epmutex across the loop check and the insert in this case, in
+         * order to prevent two separate inserts from racing and each doing the
+         * insert "at the same time" such that ep_loop_check passes on both
+         * before either one does the insert, thereby creating a cycle.
+         */
+        if (unlikely(is_file_epoll(tfile) && op == EPOLL_CTL_ADD)) {
+                mutex_lock(&epmutex);
+                did_lock_epmutex = 1;
+                error = -ELOOP;
+                if (ep_loop_check(ep, tfile) != 0)
+                        goto error_tgt_fput;
+        }
+
+
         mutex_lock(&ep->mtx);
 
         /*
@@ -1312,6 +1444,9 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
         mutex_unlock(&ep->mtx);
 
 error_tgt_fput:
+        if (unlikely(did_lock_epmutex))
+                mutex_unlock(&epmutex);
+
         fput(tfile);
 error_fput:
         fput(file);
@@ -1431,6 +1566,12 @@ static int __init eventpoll_init(void)
                 EP_ITEM_COST;
         BUG_ON(max_user_watches < 0);
 
+        /*
+         * Initialize the structure used to perform epoll file descriptor
+         * inclusion loops checks.
+         */
+        ep_nested_calls_init(&poll_loop_ncalls);
+
         /* Initialize the structure used to perform safe poll wait head wake ups */
         ep_nested_calls_init(&poll_safewake_ncalls);