Re: Fix crash when ptrace poking hugepage areas

[Andrew Morton <akpm@osdl.org>]

David Gibson <david@gibson.dropbear.id.au> wrote:
>
> Bill, does this look like the correct fix for the problem to you?  If
> so, please apply Andrew.
> 
> set_page_dirty() will not cope with being handed a page * which is
> part of a compound page, but not the master page in that compound
> page.  This case can occur via access_process_vm() if you attempt to
> write to another process's hugepage memory area using ptrace()
> (causing an oops or hang).
> 
> This patch fixes the bug by first resolving the page * to the compound
> page's master page.

We already have to handle this situation for direct-io read()s into
hugepages.  bio_set_pages_dirty() does

		if (page && !PageCompound(page))
			set_page_dirty_lock(page);

It's such a rare case that it's probably best to continue to do this in the
caller rather than in the callee.  That's access_process_vm().

Unless there's a reason why we actually want the compound page to be marked
dirty?  If there is, then direct-io has a problem.