Re: fs: out of bounds on stack in iov_iter_advance

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Aug 17, 2015 at 12:18:12PM +0300, Andrey Ryabinin wrote:

> This bug is similar to recently found bug in 9p: http://thread.gmane.org/gmane.linux.kernel/1931799/focus=1936542

Ow.  For those who'd missed that fun: the bug in question had turned out to
be caused by improper reuse of request ids, _not_ in the call chain of
the triggering syscall.

> 	if (!retval) {
> 		struct iov_iter data = *iter;
> 		retval = mapping->a_ops->direct_IO(iocb, &data, pos);
> 	}
> 
> 	if (retval > 0) {
> 		*ppos = pos + retval;
> 		iov_iter_advance(iter, retval);
> 
> 
> So either filemap_write_and_wait_range()
	Shouldn't - it's supposed to return 0 or -E...

> or mapping->a_ops->direct_IO() returned more
> than 'count'.

	Was there DAX involved?  ->direct_IO() in there is blkdev_direct_IO(),
which takes rather different paths in those cases...

> > Also too the file and line number
> > (lib/iov_iter.c:511) are completely useless because of inlining,
> > though that's not kasan's fault.

Might make sense to slap
	if (WARN_ON(size > iov_iter_count(i)))
		print size and *i
and see if it triggers...