RE: [PATCH v2] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

[David Laight <David.Laight@ACULAB.COM>]

From: David Hildenbrand
> Sent: 09 August 2022 21:57
...

These two functions seem to contain a lot of the same tests.
They also seem a bit large for 'inline'.

> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> +/* FOLL_FORCE can write to even unwritable PTEs in COW mappings. */
> +static inline bool can_follow_write_pte(pte_t pte, struct page *page,
> +					struct vm_area_struct *vma,
> +					unsigned int flags)
>  {
> -	return pte_write(pte) ||
> -		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> +	/* If the pte is writable, we can write to the page. */
> +	if (pte_write(pte))
> +		return true;
> +
> +	/* Maybe FOLL_FORCE is set to override it? */
> +	if (!(flags & FOLL_FORCE))
> +		return false;
> +
> +	/* But FOLL_FORCE has no effect on shared mappings */
> +	if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
> +		return false;
> +
> +	/* ... or read-only private ones */
> +	if (!(vma->vm_flags & VM_MAYWRITE))
> +		return false;
> +
> +	/* ... or already writable ones that just need to take a write fault */
> +	if (vma->vm_flags & VM_WRITE)
> +		return false;
> +
> +	/*
> +	 * See can_change_pte_writable(): we broke COW and could map the page
> +	 * writable if we have an exclusive anonymous page ...
> +	 */
> +	if (!page || !PageAnon(page) || !PageAnonExclusive(page))
> +		return false;
> +
> +	/* ... and a write-fault isn't required for other reasons. */
> +	if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
> +		return false;
> +	return !userfaultfd_pte_wp(vma, pte);
>  }
...
> -static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
> +/* FOLL_FORCE can write to even unwritable PMDs in COW mappings. */
> +static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
> +					struct vm_area_struct *vma,
> +					unsigned int flags)
>  {
> -	return pmd_write(pmd) ||
> -	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
> +	/* If the pmd is writable, we can write to the page. */
> +	if (pmd_write(pmd))
> +		return true;
> +
> +	/* Maybe FOLL_FORCE is set to override it? */
> +	if (!(flags & FOLL_FORCE))
> +		return false;
> +
> +	/* But FOLL_FORCE has no effect on shared mappings */
> +	if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
> +		return false;
> +
> +	/* ... or read-only private ones */
> +	if (!(vma->vm_flags & VM_MAYWRITE))
> +		return false;
> +
> +	/* ... or already writable ones that just need to take a write fault */
> +	if (vma->vm_flags & VM_WRITE)
> +		return false;
> +
> +	/*
> +	 * See can_change_pte_writable(): we broke COW and could map the page
> +	 * writable if we have an exclusive anonymous page ...
> +	 */
> +	if (!page || !PageAnon(page) || !PageAnonExclusive(page))
> +		return false;
> +
> +	/* ... and a write-fault isn't required for other reasons. */
> +	if (vma_soft_dirty_enabled(vma) && !pmd_soft_dirty(pmd))
> +		return false;
> +	return !userfaultfd_huge_pmd_wp(vma, pmd);
>  }

Perhaps only the initial call (common success path?) should
be inlined?
With the flags and vma tests being moved to an inline helper.

	David

-
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