Out-of-bounds access when hartid >= NR_CPUS

[Heiko Stübner]

Am Donnerstag, 28. Oktober 2021, 17:09:44 CEST schrieb Palmer Dabbelt:
> On Wed, 27 Oct 2021 16:34:15 PDT (-0700), atishp at atishpatra.org wrote:
> > On Tue, Oct 26, 2021 at 2:34 AM Heiko Stübner <heiko at sntech.de> wrote:
> >>
> >> Am Dienstag, 26. Oktober 2021, 10:57:26 CEST schrieb 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.
> >>
> >> and they have the set_cpu_logical_map construct to map hwids
> >> to a continuous list of cpu-ids.
> >>
> >> So with hartids not being necessarily continuous this looks like
> >> riscv would need a similar mechanism.
> >>
> >
> > RISC-V already has a similar mechanism cpuid_to_hartid_map. Logical
> > cpu ids are continuous
> > while hartid can be sparse.
> >
> > The issue here is that __cpu_up_stack/task_pointer are per hart but
> > array size depends on the NR_CPUs
> > which represents the logical CPU.
> >
> > That's why, having a maximum number of hartids defined in config will
> > be helpful.
> 
> I don't understand why we'd have both: if we can't find a CPU number for 
> a hart, then all we can do is just leave it offline.  Wouldn't it be 
> simpler to just rely on NR_CPUS?  We'll need to fix the crashes on 
> overflows either way.

I'd think so.

The mhartid register is xlen big and as the spec says they don't have to be
contiguously, so it looks like hw-designers could adopt really "creative"
numbering.

So having a NR_HARTS won't really help, because there could be huge
gaps in numbering at some point.