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

[Alexandre Ghiti]

On Mon, Dec 11, 2023 at 9:41 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> On Mon, Dec 11, 2023 at 1:52 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> >
> > 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?
> Thx for the reply!
>
> The sfence.vma is too expensive, so the situation is tricky. See the
> arm64 commit: 7f0b1bf04511 ("arm64: Fix barriers used for page table
> modifications"), which is similar. That is, linux assumes invalid pte
> won't get into TLB. Think about memory hotplug:
>
> mm/sparse.c: sparse_add_section() {
> ...
>         memmap = section_activate(nid, start_pfn, nr_pages, altmap, pgmap);
>         if (IS_ERR(memmap))
>                 return PTR_ERR(memmap);
>
>         /*
>          * Poison uninitialized struct pages in order to catch invalid flags
>          * combinations.
>          */
>         page_init_poison(memmap, sizeof(struct page) * nr_pages);
> ...
> }
> The section_activate would use set_pte to setup vmemmap, and
> page_init_poison would access these pages' struct.

So I think the generic code must be fixed by adding a
flush_cache_vmap() in vmemmap_populate_range() or similar: several
architectures implement flush_cache_vmap() because they need to do
"something" after a new mapping is established, so vmemmap should not
be any different.

>
> That means:
> sd t0, (a0) // a0 = struct page's pte address, pteval is changed from
> invalid to valid
>  ...
> lw/sw t1, (a1) // a1 = va of struct page
>
> If the lw/sw instruction is executed speculatively before the set_pte,
> we need a fence to prevent this.

Yes I agree, but to me we need the fence property of sfence.vma to
make sure the PTW sees the new pte, unless I'm mistaken and something
in the privileged specification states that a fence is enough?

>
> >
> > Thanks,
> >
> > Alex
> >
> > >  }
> > >
> > >  void flush_icache_pte(pte_t pte);
> > > --
> > > 2.40.1
> > >
>
>
>
> --
> Best Regards
>  Guo Ren