Re: [PATCH] futexs: fix infinite loop in get_futex_key on huge page

[Ingo Molnar <mingo@elte.hu>]

* Sonny Rao <sonnyrao@us.ibm.com> wrote:

> get_futex_key() can infinitely loop if it is called on a virtual 
> address that is within a huge page but not aligned to the 
> beginning of that page.  The call to get_user_pages_fast will 
> return the struct page for a sub-page within the huge page and the 
> check for page->mapping will always fail.
> 
> The fix is to call compound_head on the page before checking that 
> it's mapped.
> 
> Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: stable@kernel.org
> 
> Index: linux-2.6.31-rc2/kernel/futex.c
> ===================================================================
> --- linux-2.6.31-rc2.orig/kernel/futex.c	2009-07-10 17:45:46.181084475 -0500
> +++ linux-2.6.31-rc2/kernel/futex.c	2009-07-10 17:46:47.345084062 -0500
> @@ -247,6 +247,7 @@
>  	if (err < 0)
>  		return err;
>  
> +	page = compound_head(page);
>  	lock_page(page);
>  	if (!page->mapping) {
>  		unlock_page(page);

Nice catch! Applied it to tip:core/urgent - Thomas, do you agree 
with the fix?

	Ingo