prevent race conditions with uvwasi_fd_close()

uvwasi_fd_close() performed the following operations:

- lock the file descriptor mutex
- close the file
- release the file descriptor mutex
- call the file table's remove() function

Once the fd's mutex is released, another thread could acquire it
before the fd is removed from the file table. If this happens,
remove() could destroy a held mutex.

This commit updates uvwasi_fd_close() to perform the entire
sequence while holding the file table's lock, preventing new
acquisitions of the fd's mutex.

Fixes: #88

Author: cjihrig

include/fd_table.h

@@ -56,9 +56,9 @@ uvwasi_errno_t uvwasi_fd_table_get_nolock(struct uvwasi_fd_table_t* table,
                                           struct uvwasi_fd_wrap_t** wrap,
                                           uvwasi_rights_t rights_base,
                                           uvwasi_rights_t rights_inheriting);
-uvwasi_errno_t uvwasi_fd_table_remove(struct uvwasi_s* uvwasi,
-                                      struct uvwasi_fd_table_t* table,
-                                      const uvwasi_fd_t id);
+uvwasi_errno_t uvwasi_fd_table_remove_nolock(struct uvwasi_s* uvwasi,
+                                             struct uvwasi_fd_table_t* table,
+                                             const uvwasi_fd_t id);
 uvwasi_errno_t uvwasi_fd_table_renumber(struct uvwasi_s* uvwasi,
                                         struct uvwasi_fd_table_t* table,
                                         const uvwasi_fd_t dst,

src/fd_table.c

@@ -306,37 +306,27 @@ uvwasi_errno_t uvwasi_fd_table_get_nolock(struct uvwasi_fd_table_t* table,
 }
 
 
-uvwasi_errno_t uvwasi_fd_table_remove(uvwasi_t* uvwasi,
-                                      struct uvwasi_fd_table_t* table,
-                                      const uvwasi_fd_t id) {
+uvwasi_errno_t uvwasi_fd_table_remove_nolock(uvwasi_t* uvwasi,
+                                             struct uvwasi_fd_table_t* table,
+                                             const uvwasi_fd_t id) {
   struct uvwasi_fd_wrap_t* entry;
-  uvwasi_errno_t err;
 
   if (table == NULL)
     return UVWASI_EINVAL;
 
-  uv_rwlock_wrlock(&table->rwlock);
-
-  if (id >= table->size) {
-    err = UVWASI_EBADF;
-    goto exit;
-  }
+  if (id >= table->size)
+    return UVWASI_EBADF;
 
   entry = table->fds[id];
 
-  if (entry == NULL || entry->id != id) {
-    err = UVWASI_EBADF;
-    goto exit;
-  }
+  if (entry == NULL || entry->id != id)
+    return UVWASI_EBADF;
 
   uv_mutex_destroy(&entry->mutex);
   uvwasi__free(uvwasi, entry);
   table->fds[id] = NULL;
   table->used--;
-  err = UVWASI_ESUCCESS;
-exit:
-  uv_rwlock_wrunlock(&table->rwlock);
-  return err;
+  return UVWASI_ESUCCESS;
 }
 
 

src/uvwasi.c

@@ -878,18 +878,26 @@ uvwasi_errno_t uvwasi_fd_close(uvwasi_t* uvwasi, uvwasi_fd_t fd) {
   if (uvwasi == NULL)
     return UVWASI_EINVAL;
 
-  err = uvwasi_fd_table_get(&uvwasi->fds, fd, &wrap, 0, 0);
+  uvwasi_fd_table_lock(&uvwasi->fds);
+
+  err = uvwasi_fd_table_get_nolock(&uvwasi->fds, fd, &wrap, 0, 0);
   if (err != UVWASI_ESUCCESS)
-    return err;
+    goto exit;
 
   r = uv_fs_close(NULL, &req, wrap->fd, NULL);
   uv_mutex_unlock(&wrap->mutex);
   uv_fs_req_cleanup(&req);
 
-  if (r != 0)
-    return uvwasi__translate_uv_error(r);
+  if (r != 0) {
+    err = uvwasi__translate_uv_error(r);
+    goto exit;
+  }
+
+  err = uvwasi_fd_table_remove_nolock(uvwasi, &uvwasi->fds, fd);
 
-  return uvwasi_fd_table_remove(uvwasi, &uvwasi->fds, fd);
+exit:
+  uvwasi_fd_table_unlock(&uvwasi->fds);
+  return err;
 }