Re: [PATCH 3/6] mm: kfence: make kfence_protect_page() void

[Marco Elver]

On Tue, 28 Mar 2023 at 11:58, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> The arch_kfence_init_pool() make sure kfence pool is mapped with base page
> size (e.g. 4KB), so the following PTE lookup in kfence_protect_page() will
> always succeed. Then there is no way to stop kfence_protect_page() always
> returning true, so make it void to simplify the code.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  arch/arm/include/asm/kfence.h     |   4 +-
>  arch/arm64/include/asm/kfence.h   |   4 +-
>  arch/parisc/include/asm/kfence.h  |   7 +-
>  arch/powerpc/include/asm/kfence.h |   8 +--
>  arch/riscv/include/asm/kfence.h   |   4 +-
>  arch/s390/include/asm/kfence.h    |   3 +-
>  arch/x86/include/asm/kfence.h     |   9 +--
>  mm/kfence/core.c                  | 142 +++++++++++++++++---------------------
>  8 files changed, 73 insertions(+), 108 deletions(-)
>
> diff --git a/arch/arm/include/asm/kfence.h b/arch/arm/include/asm/kfence.h
> index 7980d0f2271f..c30a5f8125e8 100644
> --- a/arch/arm/include/asm/kfence.h
> +++ b/arch/arm/include/asm/kfence.h
> @@ -43,11 +43,9 @@ static inline bool arch_kfence_init_pool(void)
>         return true;
>  }
>
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         set_memory_valid(addr, 1, !protect);
> -
> -       return true;
>  }
>
>  #endif /* __ASM_ARM_KFENCE_H */
> diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
> index a81937fae9f6..7717c6d98b6f 100644
> --- a/arch/arm64/include/asm/kfence.h
> +++ b/arch/arm64/include/asm/kfence.h
> @@ -12,11 +12,9 @@
>
>  static inline bool arch_kfence_init_pool(void) { return true; }
>
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         set_memory_valid(addr, 1, !protect);
> -
> -       return true;
>  }
>
>  #ifdef CONFIG_KFENCE
> diff --git a/arch/parisc/include/asm/kfence.h b/arch/parisc/include/asm/kfence.h
> index 6259e5ac1fea..290792009315 100644
> --- a/arch/parisc/include/asm/kfence.h
> +++ b/arch/parisc/include/asm/kfence.h
> @@ -19,13 +19,10 @@ static inline bool arch_kfence_init_pool(void)
>  }
>
>  /* Protect the given page and flush TLB. */
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         pte_t *pte = virt_to_kpte(addr);
>
> -       if (WARN_ON(!pte))
> -               return false;
> -
>         /*
>          * We need to avoid IPIs, as we may get KFENCE allocations or faults
>          * with interrupts disabled.
> @@ -37,8 +34,6 @@ static inline bool kfence_protect_page(unsigned long addr, bool protect)
>                 set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
>
>         flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -
> -       return true;
>  }
>
>  #endif /* _ASM_PARISC_KFENCE_H */
> diff --git a/arch/powerpc/include/asm/kfence.h b/arch/powerpc/include/asm/kfence.h
> index 6fd2b4d486c5..9d8502a7d0a4 100644
> --- a/arch/powerpc/include/asm/kfence.h
> +++ b/arch/powerpc/include/asm/kfence.h
> @@ -21,16 +21,14 @@ static inline bool arch_kfence_init_pool(void)
>  }
>
>  #ifdef CONFIG_PPC64
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         struct page *page = virt_to_page(addr);
>
>         __kernel_map_pages(page, 1, !protect);
> -
> -       return true;
>  }
>  #else
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         pte_t *kpte = virt_to_kpte(addr);
>
> @@ -40,8 +38,6 @@ static inline bool kfence_protect_page(unsigned long addr, bool protect)
>         } else {
>                 pte_update(&init_mm, addr, kpte, 0, _PAGE_PRESENT, 0);
>         }
> -
> -       return true;
>  }
>  #endif
>
> diff --git a/arch/riscv/include/asm/kfence.h b/arch/riscv/include/asm/kfence.h
> index d887a54042aa..1299f47170b5 100644
> --- a/arch/riscv/include/asm/kfence.h
> +++ b/arch/riscv/include/asm/kfence.h
> @@ -46,7 +46,7 @@ static inline bool arch_kfence_init_pool(void)
>         return true;
>  }
>
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         pte_t *pte = virt_to_kpte(addr);
>
> @@ -56,8 +56,6 @@ static inline bool kfence_protect_page(unsigned long addr, bool protect)
>                 set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
>
>         flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -
> -       return true;
>  }
>
>  #endif /* _ASM_RISCV_KFENCE_H */
> diff --git a/arch/s390/include/asm/kfence.h b/arch/s390/include/asm/kfence.h
> index d55ba878378b..6d7b3632d79c 100644
> --- a/arch/s390/include/asm/kfence.h
> +++ b/arch/s390/include/asm/kfence.h
> @@ -33,10 +33,9 @@ static __always_inline void kfence_split_mapping(void)
>  #endif
>  }
>
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
>         __kernel_map_pages(virt_to_page(addr), 1, !protect);
> -       return true;
>  }
>
>  #endif /* _ASM_S390_KFENCE_H */
> diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h
> index ff5c7134a37a..6ffd4a078a71 100644
> --- a/arch/x86/include/asm/kfence.h
> +++ b/arch/x86/include/asm/kfence.h
> @@ -38,13 +38,9 @@ static inline bool arch_kfence_init_pool(void)
>  }
>
>  /* Protect the given page and flush TLB. */
> -static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +static inline void kfence_protect_page(unsigned long addr, bool protect)
>  {
> -       unsigned int level;
> -       pte_t *pte = lookup_address(addr, &level);
> -
> -       if (WARN_ON(!pte || level != PG_LEVEL_4K))
> -               return false;
> +       pte_t *pte = virt_to_kpte(addr);

This WARN and bailing here has helped us catch an issue early before
[1] - and because KFENCE ought to be enabled as a debugging tool, the
philosophy is to be failure tolerant and not crash the system here,
hence the "return false".

[1] https://lore.kernel.org/lkml/Y3bCV6VckVUEF7Pq@xxxxxxxxxxxxxxxx/

We're relying on the architecture doing the "right thing", but it's
not entirely unlikely that the arch ends up doing the wrong thing due
to some bug like above (i.e. arch_kfence_init_pool() is faulty).

Nack.