linux-kernel - Re: Linux 4.9.93

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Date:   Mon, 9 Apr 2018 19:41:11 +0900
From:   Jean-Baptiste Theou <jb@...ential.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Dan Rue <dan.rue@...aro.org>,
        Mark Brown <mark.brown@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: Linux 4.9.93

On Mon, 9 Apr 2018 11:07:41 +0100
Mark Rutland <mark.rutland@....com> wrote:

> On Mon, Apr 09, 2018 at 06:57:51PM +0900, Jean-Baptiste Theou wrote:
> > On Mon, 9 Apr 2018 11:49:37 +0200
> > Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> >   
> > > On 9 April 2018 at 11:30, Greg KH <gregkh@...uxfoundation.org> wrote:  
> > > > On Mon, Apr 09, 2018 at 06:05:34PM +0900, Jean-Baptiste Theou wrote:    
> > > >> Hi,
> > > >>
> > > >> After this patchset, a kernel built with CFI fails. Disabling
> > > >> UNMAP_KERNEL_AT_EL0 fix the issue obviously.    
> > > 
> > > How does one 'build a kernel with CFI' for arm64?  
> > 
> > From Google work on Android-4.9
> > 
> > https://android.googlesource.com/kernel/common/+/00a195e7c0752ff5d65c9caadfbcc226270ca232
> > 
> > I am not sure what is the plan on their side to upstream (Greg?), but definitely
> > useful to isolate actual issues.
> >   
> > > > Is this a "clean" 4.9.93 tree or a "4.9.93 merged into
> > > > android-common-4.9?  
> > 
> > It's a "clean 4.9.93" + whatever is needed for Clang/CFI support
> > 
> > My take is that CFI doesn't like 
> > 
> >  * void __kpti_install_ng_mappings(int cpu, int num_cpus, phys_addr_t swapper)
> > 
> > and 
> > 
> > remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
> > 
> > Maybe just flag this function to not use CFI? I remember that Sami Tolvanen did
> > similar changes.  
> 
> From a quick scan, it looks like CFI uses shadow memory for function
> prologues. Since we're taking the PA of a function pointer, presumably
> this no longer maps to valid shadow.
> 
> I'd expect the same to apply to uses of cpu_replace_ttbr1(), but it
> looks like the only user of that is marked as __init, and that patch
> adds __nocfi to __init functions.
> 
> So you probably need to mark kpti_install_ng_mappings() as __nocfi.
>  
> > I know it's a bit out of context since CFI support for ARM64 is not upstream yet,
> > but unfortunate that an stable patchset trigger such failures.  
> 
> This is simply the nature of out-of-tree code.
> 
> In future, it would be very helpful if you could provide context for
> out-of-tree patches in the initial report.
> 

I can pass the initial CFI failures by tagging the function with __nocfi, but still face issues down the road.
That said, it's out of tree, so my problem.

Will investigate.

Thanks a lot for the quick support.

Best regards

> Thanks,
> Mark.