Re: [PATCH] riscv: mm: don't let GUP-fast follow entries without _PAGE_READ

[Jisheng Zhang <jszhang@kernel.org>]

On Thu, Oct 15, 2026 at 10:37:44PM -0400, Zi Yan wrote:
> riscv maps PROT_EXEC alone as PAGE_EXEC, which has
> _PAGE_EXEC but not _PAGE_READ, and a writable shadow stack entry is
> XWR = 010, which has _PAGE_WRITE but not _PAGE_READ. riscv uses the
> generic pte_access_permitted(), pmd_access_permitted() and
> pud_access_permitted(), which only check that the entry is present and,
> for a write, writable. So GUP-fast follows both kinds of entries:
> 
> - a read, e.g. from vmsplice() or an O_DIRECT write, can read an
>   execute-only page, which userspace cannot load from;
> - a write, e.g. from an O_DIRECT read, can modify a shadow stack page,
>   as pte_write() is true for it.
> 
> Slow GUP rejects both: check_vma_flags() fails a read from a VMA without
> VM_READ and a write to a VM_SHADOW_STACK VMA, unless FOLL_FORCE is set.
> 
> Fix it by requiring both _PAGE_PRESENT and _PAGE_READ in
> pte_access_permitted(), and using it for pmd_access_permitted() and
> pud_access_permitted() too.
> 
> Requiring _PAGE_PRESENT also stops GUP-fast from following a THP PMD
> invalidated by pmdp_invalidate(). x86, arm64, and powerpc do the same
> thing.
> 
> Fixes: 3f910b7a522e ("riscv: enable HAVE_FAST_GUP if MMU")
> Fixes: c68c2ef9d641 ("riscv/mm: teach pte_mkwrite to manufacture shadow stack PTEs")
> Cc: stable@vger.kernel.org
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20261015-anon_vma_uaf_fix-v1-0-7b977303b42b%40nvidia.com?part=2
> Assisted-by: LLM
> Signed-off-by: Zi Yan <ziy@nvidia.com>

Reviewed-by: Jisheng Zhang <jszhang@kernel.org>

> ---
> Sashiko found that the riscv pmd_access_permitted() added by the
> lazyfree THP discard rework [1] lets GUP-fast bypass execute-only
> protection. The generic helpers riscv uses today have the same gap, at
> the PTE level too, and the generic pmd_access_permitted() also lets
> GUP-fast follow an invalidated PMD. This patch makes GUP-fast require
> both _PAGE_PRESENT and _PAGE_READ. Requiring _PAGE_PRESENT stops it from
> following a PMD invalidated by pmdp_invalidate(), as x86, arm64, and
> powerpc already do and as the rework relies on.
> 
> While reviewing this patch, an LLM also found that riscv's
> pte_wrprotect() sets _PAGE_READ unconditionally, which turns an
> execute-only entry into a readable one [2], e.g. every execute-only

This issue can be fixed by Nickolai's patch:
https://lore.kernel.org/linux-riscv/20261017162229.796020-1-nickolai@csail.mit.edu/

> hugetlb mapping via make_huge_pte(). That needs a separate fix.
> 
> Link: https://lore.kernel.org/all/20261015-anon_vma_uaf_fix-v1-0-7b977303b42b@nvidia.com/ [1]
> Link: https://elixir.bootlin.com/linux/v7.3-rc7/source/arch/riscv/include/asm/pgtable.h#L409 [2]
> ---
>  arch/riscv/include/asm/pgtable.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 40b1ed4f3ea89..f46b742d6dd93 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -372,6 +372,21 @@ static inline int pte_write(pte_t pte)
>  	return pte_val(pte) & _PAGE_WRITE;
>  }
>  
> +/*
> + * GUP-fast only follows entries that are present and have _PAGE_READ,
> + * and leaves the rest to slow GUP, which checks the VMA: an execute-only
> + * entry and a writable shadow stack entry (XWR = 010) lack _PAGE_READ, a
> + * PROT_NONE or invalidated entry lacks _PAGE_PRESENT. Any other writable
> + * entry is readable too.
> + */
> +#define pte_access_permitted pte_access_permitted
> +static inline bool pte_access_permitted(pte_t pte, bool write)
> +{
> +	const unsigned long need = _PAGE_PRESENT | _PAGE_READ;
> +
> +	return (pte_val(pte) & need) == need && (!write || pte_write(pte));
> +}
> +
>  static inline int pte_exec(pte_t pte)
>  {
>  	return pte_val(pte) & _PAGE_EXEC;
> @@ -820,12 +835,24 @@ static inline int pmd_write(pmd_t pmd)
>  	return pte_write(pmd_pte(pmd));
>  }
>  
> +#define pmd_access_permitted pmd_access_permitted
> +static inline bool pmd_access_permitted(pmd_t pmd, bool write)
> +{
> +	return pte_access_permitted(pmd_pte(pmd), write);
> +}
> +
>  #define pud_write pud_write
>  static inline int pud_write(pud_t pud)
>  {
>  	return pte_write(pud_pte(pud));
>  }
>  
> +#define pud_access_permitted pud_access_permitted
> +static inline bool pud_access_permitted(pud_t pud, bool write)
> +{
> +	return pte_access_permitted(pud_pte(pud), write);
> +}
> +
>  #define pmd_dirty pmd_dirty
>  static inline int pmd_dirty(pmd_t pmd)
>  {
> 
> ---
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
> change-id: 20261015-riscv-gup-read-fix-d657d67d350a
> 
> Best regards,
> --  
> Yan, Zi
> 
> 
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