Re: [PATCH] riscv: pgtable: Enhance set_pte to prevent OoO risk

[Alexandre Ghiti]

Hi Guo,

On Fri, Dec 8, 2023 at 4:10 PM <guoren@xxxxxxxxxx> wrote:
>
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> When changing from an invalid pte to a valid one for a kernel page,
> there is no need for tlb_flush. It's okay for the TSO memory model, but
> there is an OoO risk for the Weak one. eg:
>
> sd t0, (a0) // a0 = pte address, pteval is changed from invalid to valid
> ...
> ld t1, (a1) // a1 = va of above pte
>
> If the ld instruction is executed speculatively before the sd
> instruction. Then it would bring an invalid entry into the TLB, and when
> the ld instruction retired, a spurious page fault occurred. Because the
> vmemmap has been ignored by vmalloc_fault, the spurious page fault would
> cause kernel panic.
>
> This patch was inspired by the commit: 7f0b1bf04511 ("arm64: Fix barriers
> used for page table modifications"). For RISC-V, there is no requirement
> in the spec to guarantee all tlb entries are valid and no requirement to
> PTW filter out invalid entries. Of course, micro-arch could give a more
> robust design, but here, use a software fence to guarantee.
>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/pgtable.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 294044429e8e..2fae5a5438e0 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -511,6 +511,13 @@ static inline int pte_same(pte_t pte_a, pte_t pte_b)
>  static inline void set_pte(pte_t *ptep, pte_t pteval)
>  {
>         *ptep = pteval;
> +
> +       /*
> +        * Only if the new pte is present and kernel, otherwise TLB
> +        * maintenance or update_mmu_cache() have the necessary barriers.
> +        */
> +       if (pte_val(pteval) & (_PAGE_PRESENT | _PAGE_GLOBAL))
> +               RISCV_FENCE(rw,rw);

Only a sfence.vma can guarantee that the PTW actually sees a new
mapping, a fence is not enough. That being said, new kernel mappings
(vmalloc ones) are correctly handled in the kernel by using
flush_cache_vmap(). Did you observe something that this patch fixes?

Thanks,

Alex

>  }
>
>  void flush_icache_pte(pte_t pte);
> --
> 2.40.1
>