From cdc82807176728ac31fdf12bb36a2ce72783af45 Mon Sep 17 00:00:00 2001
From: cjihrig <cjihrig@gmail.com>
Date: Tue, 21 Jan 2020 23:08:38 -0500
Subject: [PATCH] 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: https://github.com/cjihrig/uvwasi/issues/88
---
 include/fd_table.h |  6 +++---
 src/fd_table.c     | 26 ++++++++------------------
 src/uvwasi.c       | 18 +++++++++++++-----
 3 files changed, 24 insertions(+), 26 deletions(-)

diff --git a/include/fd_table.h b/include/fd_table.h
index 9d88628..fa8a44e 100644
--- a/include/fd_table.h
+++ b/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,
diff --git a/src/fd_table.c b/src/fd_table.c
index c15ea09..bc32f4d 100644
--- a/src/fd_table.c
+++ b/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;
 }
 
 
diff --git a/src/uvwasi.c b/src/uvwasi.c
index 9fa4db8..53b7699 100644
--- a/src/uvwasi.c
+++ b/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;
 }