Re: [PATCH 4.4 099/190] arm64: futex: Fix undefined behaviour with FUTEX_OP_OPARG_SHIFT usage

[Greg Kroah-Hartman]

On Wed, May 16, 2018 at 12:15:36AM +0100, Ben Hutchings wrote:
> On Wed, 2018-04-11 at 20:35 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > > From: Will Deacon <will.deacon@xxxxxxx>
> > 
> > 
> > [ Upstream commit 5f16a046f8e144c294ef98cd29d9458b5f8273e5 ]
> > 
> > FUTEX_OP_OPARG_SHIFT instructs the futex code to treat the 12-bit oparg
> > field as a shift value, potentially leading to a left shift value that
> > is negative or with an absolute value that is significantly larger then
> > the size of the type. UBSAN chokes with:
> [...]
> > Whilst I think this catches all of the issues, I'd much prefer to remove
> > this stuff, as I think it's unused and the bugs are copy-pasted between
> > a bunch of architectures.
> [...]
> 
> Indeed.  That more complete fix was done upstream by:
> 
> commit 30d6e0a4190d37740e9447e4e4815f06992dd8c3
> Author: Jiri Slaby <jslaby@xxxxxxx>
> Date:   Thu Aug 24 09:31:05 2017 +0200
> 
>     futex: Remove duplicated code and fix undefined behaviour
> 
> It's a bit big for stable - though most of the changes are deletions. 
> What do you think?

I think it makes a lot of sense, I've done the backport now, thanks.

greg k-h