Out-of-bounds access when hartid >= NR_CPUS

[Geert Uytterhoeven]

Hi Heiko,

On Tue, Oct 26, 2021 at 10:53 AM Heiko Stübner <heiko at sntech.de> wrote:
> Am Dienstag, 26. Oktober 2021, 08:44:31 CEST schrieb Geert Uytterhoeven:
> > On Tue, Oct 26, 2021 at 2:37 AM Ron Economos <re at w6rz.net> wrote:
> > > On 10/25/21 8:54 AM, Geert Uytterhoeven wrote:
> > > > When booting a kernel with CONFIG_NR_CPUS=4 on Microchip PolarFire,
> > > > the 4th CPU either fails to come online, or the system crashes.
> > > >
> > > > This happens because PolarFire has 5 CPU cores: hart 0 is an e51,
> > > > and harts 1-4 are u54s, with the latter becoming CPUs 0-3 in Linux:
> > > >    - unused core has hartid 0 (sifive,e51),
> > > >    - processor 0 has hartid 1 (sifive,u74-mc),
> > > >    - processor 1 has hartid 2 (sifive,u74-mc),
> > > >    - processor 2 has hartid 3 (sifive,u74-mc),
> > > >    - processor 3 has hartid 4 (sifive,u74-mc).
> > > >
> > > > I assume the same issue is present on the SiFive fu540 and fu740
> > > > SoCs, but I don't have access to these.  The issue is not present
> > > > on StarFive JH7100, as processor 0 has hartid 1, and processor 1 has
> > > > hartid 0.
> > > >
> > > > arch/riscv/kernel/cpu_ops.c has:
> > > >
> > > >      void *__cpu_up_stack_pointer[NR_CPUS] __section(".data");
> > > >      void *__cpu_up_task_pointer[NR_CPUS] __section(".data");
> > > >
> > > >      void cpu_update_secondary_bootdata(unsigned int cpuid,
> > > >                                         struct task_struct *tidle)
> > > >      {
> > > >              int hartid = cpuid_to_hartid_map(cpuid);
> > > >
> > > >              /* Make sure tidle is updated */
> > > >              smp_mb();
> > > >              WRITE_ONCE(__cpu_up_stack_pointer[hartid],
> > > >                         task_stack_page(tidle) + THREAD_SIZE);
> > > >              WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
> > > >
> > > > The above two writes cause out-of-bound accesses beyond
> > > > __cpu_up_{stack,pointer}_pointer[] if hartid >= CONFIG_NR_CPUS.
> > > >
> > > >      }

> > https://riscv.org/wp-content/uploads/2017/05/riscv-privileged-v1.10.pdf
> > says:
> >
> >     Hart IDs might not necessarily be numbered contiguously in a
> >     multiprocessor system, but at least one hart must have a hart
> >     ID of zero.
> >
> > Which means indexing arrays by hart ID is a no-go?
>
> Isn't that also similar on aarch64?
>
> On a rk3399 you get 0-3 and 100-101 and with the paragraph above
> something like this could very well exist on some riscv cpu too I guess.

Yes, it looks like hart IDs are similar to MPIDRs on ARM.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds