cifs: fix the race in cifs_writev()

O_APPEND handling there hadn't been completely fixed by Pavel's
patch; it checks the right value, but it's racy - we can't really
do that until i_mutex has been taken.

Fix by switching to __generic_file_aio_write() (open-coding
generic_file_aio_write(), actually) and pulling mutex_lock() above
inode_size_read().

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 18 insertions, 5 deletions

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 3443b8f8e1c0..5bac2763c450 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2579,19 +2579,32 @@ cifs_writev(struct kiocb *iocb, const struct iovec *iov,
         struct cifsInodeInfo *cinode = CIFS_I(inode);
         struct TCP_Server_Info *server = tlink_tcon(cfile->tlink)->ses->server;
         ssize_t rc = -EACCES;
-        loff_t lock_pos = pos;
+        loff_t lock_pos = iocb->ki_pos;
 
-        if (file->f_flags & O_APPEND)
-                lock_pos = i_size_read(inode);
         /*
          * We need to hold the sem to be sure nobody modifies lock list
          * with a brlock that prevents writing.
          */
         down_read(&cinode->lock_sem);
+        mutex_lock(&inode->i_mutex);
+        if (file->f_flags & O_APPEND)
+                lock_pos = i_size_read(inode);
         if (!cifs_find_lock_conflict(cfile, lock_pos, iov_length(iov, nr_segs),
                                      server->vals->exclusive_lock_type, NULL,
-                                     CIFS_WRITE_OP))
-                rc = generic_file_aio_write(iocb, iov, nr_segs, pos);
+                                     CIFS_WRITE_OP)) {
+                rc = __generic_file_aio_write(iocb, iov, nr_segs);
+                mutex_unlock(&inode->i_mutex);
+
+                if (rc > 0) {
+                        ssize_t err;
+
+                        err = generic_write_sync(file, iocb->ki_pos - rc, rc);
+                        if (rc < 0)
+                                rc = err;
+                }
+        } else {
+                mutex_unlock(&inode->i_mutex);
+        }
         up_read(&cinode->lock_sem);
         return rc;
 }