ext4: punch_hole should wait for DIO writers

punch_hole is the place where we have to wait for all existing writers
(writeback, aio, dio), but currently we simply flush pended end_io request
which is not sufficient. Other issue is that punch_hole performed w/o i_mutex
held which obviously result in dangerous data corruption due to
write-after-free.

This patch performs following changes:
- Guard punch_hole with i_mutex
- Recheck inode flags under i_mutex
- Block all new dio readers in order to prevent information leak caused by
  read-after-free pattern.
- punch_hole now wait for all writers in flight
  NOTE: XXX write-after-free race is still possible because new dirty pages
  may appear due to mmap(), and currently there is no easy way to stop
  writeback while punch_hole is in progress.

[ Fixed error return from ext4_ext_punch_hole() to make sure that we
  release i_mutex before returning EPERM or ETXTBUSY -- Ted ]

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 36 insertions, 17 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 232077439aa8..5920e75fc05f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4794,9 +4794,32 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
         loff_t first_page_offset, last_page_offset;
         int credits, err = 0;
 
+        /*
+         * Write out all dirty pages to avoid race conditions
+         * Then release them.
+         */
+        if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
+                err = filemap_write_and_wait_range(mapping,
+                        offset, offset + length - 1);
+
+                if (err)
+                        return err;
+        }
+
+        mutex_lock(&inode->i_mutex);
+        /* It's not possible punch hole on append only file */
+        if (IS_APPEND(inode) || IS_IMMUTABLE(inode)) {
+                err = -EPERM;
+                goto out_mutex;
+        }
+        if (IS_SWAPFILE(inode)) {
+                err = -ETXTBSY;
+                goto out_mutex;
+        }
+
         /* No need to punch hole beyond i_size */
         if (offset >= inode->i_size)
-                return 0;
+                goto out_mutex;
 
         /*
          * If the hole extends beyond i_size, set the hole
@@ -4814,33 +4837,25 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
         first_page_offset = first_page << PAGE_CACHE_SHIFT;
         last_page_offset = last_page << PAGE_CACHE_SHIFT;
 
-        /*
-         * Write out all dirty pages to avoid race conditions
-         * Then release them.
-         */
-        if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
-                err = filemap_write_and_wait_range(mapping,
-                        offset, offset + length - 1);
-
-                if (err)
-                        return err;
-        }
-
         /* Now release the pages */
         if (last_page_offset > first_page_offset) {
                 truncate_pagecache_range(inode, first_page_offset,
                                          last_page_offset - 1);
         }
 
-        /* finish any pending end_io work */
+        /* Wait all existing dio workers, newcomers will block on i_mutex */
+        ext4_inode_block_unlocked_dio(inode);
+        inode_dio_wait(inode);
         err = ext4_flush_completed_IO(inode);
         if (err)
-                return err;
+                goto out_dio;
 
         credits = ext4_writepage_trans_blocks(inode);
         handle = ext4_journal_start(inode, credits);
-        if (IS_ERR(handle))
-                return PTR_ERR(handle);
+        if (IS_ERR(handle)) {
+                err = PTR_ERR(handle);
+                goto out_dio;
+        }
 
 
         /*
@@ -4930,6 +4945,10 @@ out:
         inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
         ext4_mark_inode_dirty(handle, inode);
         ext4_journal_stop(handle);
+out_dio:
+        ext4_inode_resume_unlocked_dio(inode);
+out_mutex:
+        mutex_unlock(&inode->i_mutex);
         return err;
 }
 int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,