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
