ext4: fix an use-after-free issue about data=journal writeback mode

Our syzkaller report an use-after-free issue that accessing the freed
buffer_head on the writeback page in __ext4_journalled_writepage(). The
problem is that if there was a truncate racing with the data=journalled
writeback procedure, the writeback length could become zero and
bget_one() refuse to get buffer_head's refcount, then the truncate
procedure release buffer once we drop page lock, finally, the last
ext4_walk_page_buffers() trigger the use-after-free problem.

sync                               truncate
ext4_sync_file()
 file_write_and_wait_range()
                                   ext4_setattr(0)
                                    inode->i_size = 0
  ext4_writepage()
   len = 0
   __ext4_journalled_writepage()
    page_bufs = page_buffers(page)
    ext4_walk_page_buffers(bget_one) <- does not get refcount
                                    do_invalidatepage()
                                      free_buffer_head()
    ext4_walk_page_buffers(page_bufs) <- trigger use-after-free

After commit bdf96838aea6 ("ext4: fix race between truncate and
__ext4_journalled_writepage()"), we have already handled the racing
case, so the bget_one() and bput_one() are not needed. So this patch
simply remove these hunk, and recheck the i_size to make it safe.

Fixes: bdf96838aea6 ("ext4: fix race between truncate and __ext4_journalled_writepage()")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211225090937.712867-1-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index bca9951634d945d564dc9d9aeb27aad98d6a3c04..68070f34f0cff7baf615e30e17bd95e1d5c723cb 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1845,30 +1845,16 @@ int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
        return 0;
 }
 
-static int bget_one(handle_t *handle, struct inode *inode,
-                   struct buffer_head *bh)
-{
-       get_bh(bh);
-       return 0;
-}
-
-static int bput_one(handle_t *handle, struct inode *inode,
-                   struct buffer_head *bh)
-{
-       put_bh(bh);
-       return 0;
-}
-
 static int __ext4_journalled_writepage(struct page *page,
                                       unsigned int len)
 {
        struct address_space *mapping = page->mapping;
        struct inode *inode = mapping->host;
-       struct buffer_head *page_bufs = NULL;
        handle_t *handle = NULL;
        int ret = 0, err = 0;
        int inline_data = ext4_has_inline_data(inode);
        struct buffer_head *inode_bh = NULL;
+       loff_t size;
 
        ClearPageChecked(page);
 
@@ -1878,14 +1864,6 @@ static int __ext4_journalled_writepage(struct page *page,
                inode_bh = ext4_journalled_write_inline_data(inode, len, page);
                if (inode_bh == NULL)
                        goto out;
-       } else {
-               page_bufs = page_buffers(page);
-               if (!page_bufs) {
-                       BUG();
-                       goto out;
-               }
-               ext4_walk_page_buffers(handle, inode, page_bufs, 0, len,
-                                      NULL, bget_one);
        }
        /*
         * We need to release the page lock before we start the
@@ -1906,7 +1884,8 @@ static int __ext4_journalled_writepage(struct page *page,
 
        lock_page(page);
        put_page(page);
-       if (page->mapping != mapping) {
+       size = i_size_read(inode);
+       if (page->mapping != mapping || page_offset(page) > size) {
                /* The page got truncated from under us */
                ext4_journal_stop(handle);
                ret = 0;
@@ -1916,6 +1895,13 @@ static int __ext4_journalled_writepage(struct page *page,
        if (inline_data) {
                ret = ext4_mark_inode_dirty(handle, inode);
        } else {
+               struct buffer_head *page_bufs = page_buffers(page);
+
+               if (page->index == size >> PAGE_SHIFT)
+                       len = size & ~PAGE_MASK;
+               else
+                       len = PAGE_SIZE;
+
                ret = ext4_walk_page_buffers(handle, inode, page_bufs, 0, len,
                                             NULL, do_journal_get_write_access);
 
@@ -1936,9 +1922,6 @@ static int __ext4_journalled_writepage(struct page *page,
 out:
        unlock_page(page);
 out_no_pagelock:
-       if (!inline_data && page_bufs)
-               ext4_walk_page_buffers(NULL, inode, page_bufs, 0, len,
-                                      NULL, bput_one);
        brelse(inode_bh);
        return ret;
 }