Re: [PATCH v5 02/12] arch: export set_direct_map_valid_noflush to KVM module

[Fuad Tabba <tabba@google.com>]

Hi Patrick,

On Thu, 28 Aug 2025 at 10:39, Roy, Patrick <roypat@amazon.co.uk> wrote:
>
> Use the new per-module export functionality to allow KVM (and only KVM)
> access to set_direct_map_valid_noflush(). This allows guest_memfd to
> remove its memory from the direct map, even if KVM is built as a module.
>
> Direct map removal gives guest_memfd the same protection that
> memfd_secret enjoys, such as hardening against Spectre-like attacks
> through in-kernel gadgets.
>
> Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> ---
>  arch/arm64/mm/pageattr.c     | 1 +
>  arch/loongarch/mm/pageattr.c | 1 +
>  arch/riscv/mm/pageattr.c     | 1 +
>  arch/s390/mm/pageattr.c      | 1 +
>  arch/x86/mm/pat/set_memory.c | 1 +
>  5 files changed, 5 insertions(+)
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 04d4a8f676db..4f3cddfab9b0 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -291,6 +291,7 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>
>         return set_memory_valid(addr, nr, valid);
>  }
> +EXPORT_SYMBOL_FOR_MODULES(set_direct_map_valid_noflush, "kvm");
>
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  /*
> diff --git a/arch/loongarch/mm/pageattr.c b/arch/loongarch/mm/pageattr.c
> index f5e910b68229..d076bfd3fcbf 100644
> --- a/arch/loongarch/mm/pageattr.c
> +++ b/arch/loongarch/mm/pageattr.c
> @@ -217,6 +217,7 @@ int set_direct_map_invalid_noflush(struct page *page)
>
>         return __set_memory(addr, 1, __pgprot(0), __pgprot(_PAGE_PRESENT | _PAGE_VALID));
>  }
> +EXPORT_SYMBOL_FOR_MODULES(set_direct_map_valid_noflush, "kvm");

This should be after 'set_direct_map_valid_noflush', not 'invalid'.

With that fixed:

Reviewed-by: Fuad Tabba <tabba@google.com>

>  int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>  {
> diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
> index 3f76db3d2769..6db31040cd66 100644
> --- a/arch/riscv/mm/pageattr.c
> +++ b/arch/riscv/mm/pageattr.c
> @@ -400,6 +400,7 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>
>         return __set_memory((unsigned long)page_address(page), nr, set, clear);
>  }
> +EXPORT_SYMBOL_FOR_MODULES(set_direct_map_valid_noflush, "kvm");
>
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  static int debug_pagealloc_set_page(pte_t *pte, unsigned long addr, void *data)
> diff --git a/arch/s390/mm/pageattr.c b/arch/s390/mm/pageattr.c
> index 348e759840e7..8ffd9ef09bc6 100644
> --- a/arch/s390/mm/pageattr.c
> +++ b/arch/s390/mm/pageattr.c
> @@ -413,6 +413,7 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>
>         return __set_memory((unsigned long)page_to_virt(page), nr, flags);
>  }
> +EXPORT_SYMBOL_FOR_MODULES(set_direct_map_valid_noflush, "kvm");
>
>  bool kernel_page_present(struct page *page)
>  {
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 8834c76f91c9..87e9c7d2dcdc 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2661,6 +2661,7 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>
>         return __set_pages_np(page, nr);
>  }
> +EXPORT_SYMBOL_FOR_MODULES(set_direct_map_valid_noflush, "kvm");
>
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  void __kernel_map_pages(struct page *page, int numpages, int enable)
> --
> 2.50.1
>