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Subject: Re: Out-of-bounds access when hartid >= NR_CPUS
To:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>
Cc:     linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <CAMuHMdUPWOjJfJohxLJefHOrJBtXZ0xfHQt4=hXpUXnasiN+AQ@mail.gmail.com>
From:   Ron Economos <re@w6rz.net>
Message-ID: <830eda64-6e66-c61b-ceaa-57be87783b2c@w6rz.net>
Date:   Mon, 25 Oct 2021 17:37:08 -0700
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On 10/25/21 8:54 AM, Geert Uytterhoeven wrote:

> Hi all,
>
> 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.
>
>      }
>
> arch/riscv/kernel/smpboot.c:setup_smp(void) detects CPUs like this:
>
>      for_each_of_cpu_node(dn) {
>              hart = riscv_of_processor_hartid(dn);
>              if (hart < 0)
>                      continue;
>
>              if (hart == cpuid_to_hartid_map(0)) {
>                      BUG_ON(found_boot_cpu);
>                      found_boot_cpu = 1;
>                      early_map_cpu_to_node(0, of_node_to_nid(dn));
>                      continue;
>              }
>              if (cpuid >= NR_CPUS) {
>                      pr_warn("Invalid cpuid [%d] for hartid [%d]\n",
>                              cpuid, hart);
>                      break;
>              }
>
>              cpuid_to_hartid_map(cpuid) = hart;
>              early_map_cpu_to_node(cpuid, of_node_to_nid(dn));
>              cpuid++;
>      }
>
> So cpuid >= CONFIG_NR_CPUS (too many CPU cores) is already rejected.
>
> How to fix this?
>
> We could skip hartids >= NR_CPUS, but that feels strange to me, as
> you need NR_CPUS to be larger (much larger if the first usable hartid
> is a large number) than the number of CPUs used.
The Ubuntu distro config for HiFive Unmatched set this to CONFIG_NR_CPUS=8.
>
> We could store the minimum hartid, and always subtract that when
> accessing __cpu_up_{stack,pointer}_pointer[] (also in
> arch/riscv/kernel/head.S), but that means unused cores cannot be in the
> middle of the hartid range.
>
> Are hartids guaranteed to be continuous? If not, we have no choice but
> to index __cpu_up_{stack,pointer}_pointer[] by cpuid instead, which
> needs a more expensive conversion in arch/riscv/kernel/head.S.
>
> Thanks for your comments!
>
> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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
>
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