Re: [PATCH v5 19/30] arm64: add POE signal support

[Will Deacon <will@kernel.org>]

On Tue, Oct 15, 2024 at 10:59:11AM +0100, Joey Gouly wrote:
> On Mon, Oct 14, 2024 at 06:10:23PM +0100, Will Deacon wrote:
> > Kevin, Joey,
> > 
> > On Wed, Oct 09, 2024 at 03:43:01PM +0100, Will Deacon wrote:
> > > On Tue, Sep 24, 2024 at 01:27:58PM +0200, Kevin Brodsky wrote:
> > > > On 22/08/2024 17:11, Joey Gouly wrote:
> > > > > @@ -1178,6 +1237,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
> > > > >  		sme_smstop();
> > > > >  	}
> > > > >  
> > > > > +	if (system_supports_poe())
> > > > > +		write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0);
> > > > 
> > > > At the point where setup_return() is called, the signal frame has
> > > > already been written to the user stack. In other words, we write to the
> > > > user stack first, and then reset POR_EL0. This may be problematic,
> > > > especially if we are using the alternate signal stack, which the
> > > > interrupted POR_EL0 may not grant access to. In that situation uaccess
> > > > will fail and we'll end up with a SIGSEGV.
> > > > 
> > > > This issue has already been discussed on the x86 side, and as it happens
> > > > patches to reset PKRU early [1] have just landed. I don't think this is
> > > > a blocker for getting this series landed, but we should try and align
> > > > with x86. If there's no objection, I'm planning to work on a counterpart
> > > > to the x86 series (resetting POR_EL0 early during signal delivery).
> > > 
> > > Did you get a chance to work on that? It would be great to land the
> > > fixes for 6.12, if possible, so that the first kernel release with POE
> > > support doesn't land with known issues.
> > 
> > Looking a little more at this, I think we have quite a weird behaviour
> > on arm64 as it stands. It looks like we rely on the signal frame to hold
> > the original POR_EL0 so, if for some reason we fail to allocate space
> > for the POR context, I think we'll return back from the signal with
> > POR_EL0_INIT. That seems bad?
> 
> If we don't allocate space for POR_EL0, I think the program recieves SIGSGEV?
> 
> setup_sigframe_layout()
>         if (system_supports_poe()) {
>                 err = sigframe_alloc(user, &user->poe_offset,
>                                      sizeof(struct poe_context));
>                 if (err)
>                         return err;
>         }
> 
> Through get_sigframe() and setup_rt_frame(), that eventually hets here:
> 
> handle_signal()
> 	ret = setup_rt_frame(usig, ksig, oldset, regs);
> 
> 	[..]
> 
>         signal_setup_done(ret, ksig, test_thread_flag(TIF_SINGLESTEP));
> 
> void signal_setup_done(int failed, struct ksignal *ksig, int stepping)                                                                                                                         
> {                                                                                                                                                                                              
>         if (failed)                                                                                                                                                                            
>                 force_sigsegv(ksig->sig);                                                                                                                                                      
>         else                                                                                                                                                                                   
>                 signal_delivered(ksig, stepping);                                                                                                                                              
> }  
> 
> So I think it's "fine"?

Ah, yes, sorry about that. I got confused by the conditional push in
setup_sigframe():

	if (system_supports_poe() && err == 0 && user->poe_offset) {
		...

which gives the wrong impression that the POR is somehow optional, even
if the CPU supports POE. So we should drop that check of
'user->poe_offset' as it cannot be NULL here.

We also still need to resolve Kevin's concern, which probably means
keeping the thread's original POR around someplace.

Will