Patch "vfs,mm: fix a dead loop in truncate_inode_pages_range()" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

vfs,mm: fix a dead loop in truncate_inode_pages_range()

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
vfs-mm-fix-a-dead-loop-in-truncate_inode_pages_range.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wei Fang <[email protected]>
Date: Fri, 7 Oct 2016 17:01:52 -0700
Subject: vfs,mm: fix a dead loop in truncate_inode_pages_range()

From: Wei Fang <[email protected]>

commit c2a9737f45e27d8263ff9643f994bda9bac0b944 upstream.

We triggered a deadloop in truncate_inode_pages_range() on 32 bits architecture with the test case bellow:

	...
	fd = open();
	write(fd, buf, 4096);
	preadv64(fd, &iovec, 1, 0xffffffff000);
	ftruncate(fd, 0);
	...

Then ftruncate() will not return forever.

The filesystem used in this case is ubifs, but it can be triggered on many other filesystems.

When preadv64() is called with offset=0xffffffff000, a page with index=0xffffffff will be added to the radix tree of ->mapping. Then this page can be found in ->mapping with pagevec_lookup(). After that, truncate_inode_pages_range(), which is called in ftruncate(), will fall into an infinite loop:

- find a page with index=0xffffffff, since index>=end, this page won't be truncated

- index++, and index become 0

- the page with index=0xffffffff will be found again

The data type of index is unsigned long, so index won't overflow to 0 on 64 bits architecture in this case, and the dead loop won't happen.

Since truncate_inode_pages_range() is executed with holding lock of inode->i_rwsem, any operation related with this lock will be blocked, and a hung task will happen, e.g.:

  INFO: task truncate_test:3364 blocked for more than 120 seconds.
  ...
     call_rwsem_down_write_failed+0x17/0x30
     generic_file_write_iter+0x32/0x1c0
     ubifs_write_iter+0xcc/0x170
     __vfs_write+0xc4/0x120
     vfs_write+0xb2/0x1b0
     SyS_write+0x46/0xa0

The page with index=0xffffffff added to ->mapping is useless. Fix this by checking the read position before allocating pages.

---
 mm/filemap.c |    4 ++++
 1 file changed, 4 insertions(+)

@@ -1687,6 +1687,10 @@ static ssize_t do_generic_file_read(stru
 	unsigned int prev_offset;
 	int error = 0;
 
+	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
+		return -EINVAL;
+	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
+
 	index = *ppos >> PAGE_SHIFT;
 	prev_index = ra->prev_pos >> PAGE_SHIFT;
 	prev_offset = ra->prev_pos & (PAGE_SIZE-1);

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/vfs-mm-fix-a-dead-loop-in-truncate_inode_pages_range.patch