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"?
Thanks,
Joey
