write: Validate file handle in write system call before accessing the
writable attribute.

---
 firmware/common/file.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/firmware/common/file.c b/firmware/common/file.c
index 9096587..5ef00ea 100644
--- a/firmware/common/file.c
+++ b/firmware/common/file.c
@@ -680,7 +680,9 @@ static int readwrite(int fd, void* buf, long count, bool write)
 
 ssize_t write(int fd, const void* buf, size_t count)
 {
-    if (!openfiles[fd].write) {
+    if (openfiles[fd].busy && !openfiles[fd].write) /* other cases handled by
+                                                       readwrite() */
+    {
         errno = EACCES;
         return -1;
     }
-- 
1.7.1