fuse: fix race in fuse_writepages()

The patch fixes a race between ftruncate(2), mmap-ed write and write(2):

1) An user makes a page dirty via mmap-ed write.
2) The user performs shrinking truncate(2) intended to purge the page.
3) Before fuse_do_setattr calls truncate_pagecache, the page goes to
   writeback. fuse_writepages_fill attaches a new page to FUSE_WRITE request,
   then releases the original page by end_page_writeback and unlock it.
4) fuse_do_setattr completes and successfully returns. Since now, i_mutex
   is free.
5) Ordinary write(2) extends i_size back to cover the page. Note that
   fuse_send_write_pages do wait for fuse writeback, but for another
   page->index.
6) fuse_writepages_fill attaches more pages to the request (if any), then
   fuse_writepages_send is eventually called. It is supposed to crop
   inarg->size of the request, but it doesn't because i_size has already been
   extended back.

Moving end_page_writeback behind fuse_writepages_send guarantees that
__fuse_release_nowrite (called from fuse_do_setattr) will crop inarg->size
of the request before write(2) gets the chance to extend i_size.

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

1 files changed, 16 insertions, 1 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 0bd349dd968f..cc3a6c4437e4 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1590,6 +1590,7 @@ struct fuse_fill_wb_data {
         struct fuse_req *req;
         struct fuse_file *ff;
         struct inode *inode;
+        struct page **orig_pages;
 };
 
 static void fuse_writepages_send(struct fuse_fill_wb_data *data)
@@ -1598,12 +1599,17 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
         struct inode *inode = data->inode;
         struct fuse_conn *fc = get_fuse_conn(inode);
         struct fuse_inode *fi = get_fuse_inode(inode);
+        int num_pages = req->num_pages;
+        int i;
 
         req->ff = fuse_file_get(data->ff);
         spin_lock(&fc->lock);
         list_add_tail(&req->list, &fi->queued_writes);
         fuse_flush_writepages(inode);
         spin_unlock(&fc->lock);
+
+        for (i = 0; i < num_pages; i++)
+                end_page_writeback(data->orig_pages[i]);
 }
 
 static int fuse_writepages_fill(struct page *page,
@@ -1684,7 +1690,7 @@ static int fuse_writepages_fill(struct page *page,
 
         inc_bdi_stat(page->mapping->backing_dev_info, BDI_WRITEBACK);
         inc_zone_page_state(tmp_page, NR_WRITEBACK_TEMP);
-        end_page_writeback(page);
+        data->orig_pages[req->num_pages] = page;
 
         /*
          * Protected by fc->lock against concurrent access by
@@ -1716,6 +1722,13 @@ static int fuse_writepages(struct address_space *mapping,
         data.req = NULL;
         data.ff = NULL;
 
+        err = -ENOMEM;
+        data.orig_pages = kzalloc(sizeof(struct page *) *
+                                  FUSE_MAX_PAGES_PER_REQ,
+                                  GFP_NOFS);
+        if (!data.orig_pages)
+                goto out;
+
         err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
         if (data.req) {
                 /* Ignore errors if we can write at least one page */
@@ -1725,6 +1738,8 @@ static int fuse_writepages(struct address_space *mapping,
         }
         if (data.ff)
                 fuse_file_put(data.ff, false);
+
+        kfree(data.orig_pages);
 out:
         return err;
 }