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

[Will Deacon]

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?

Will