Re: [PATCH 16/35] x86/mm: Update maybe_mkwrite() for shadow stack

Dave Hansen
2022-02-09T21:17:00Z

First of all, that changelog doesn't really explain the problem. It's
all background and no "why".

*Why* does maybe_mkwrite() take a VMA? What's the point?

> #endif /* _ASM_X86_PGTABLE_H */
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index 3481b35cb4ec..c22c8e9c37e8 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -610,6 +610,26 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
> }
> #endif
>
> +pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> +{
> + if (vma->vm_flags & VM_WRITE)
> + pte = pte_mkwrite(pte);
> + else if (vma->vm_flags & VM_SHADOW_STACK)
> + pte = pte_mkwrite_shstk(pte);
> + return pte;
> +}

First, this makes me wonder why we need pte_mkwrite() *AND*
pte_mkwrite_shstk(). Is there a difference in their behavior that matters?

Second, I don't like the copy-and-paste to make an arch-specific "hook"
for a function. This is a very good way to ensure that arch code and
generic code fork and accumulate separate bugs.

I'd much rather have this do (in generic code):

pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
if (vma->vm_flags & VM_WRITE)
pte = pte_mkwrite(pte);

pte = arch_maybe_mkwrite(pte, vma);

return pte;
+}

Actually, is there a reason the generic code could not even just add:

if (vma->vm_flags & VM_ARCH_MAYBE_MKWRITE_MASK)
pte = arch_maybe_mkwrite(pte, vma);

or heck even just the x86-specific code itself:

if (vma->vm_flags & VM_SHADOW_STACK)
pte = pte_mkwrite_shstk(pte);

with a stub defined for pte_mkwrite_shstk()?

In the end, it's just a question of whether the generic code wants
something to say "arch" or "shstk". But, I don't think we need a forked
x86 copy of these functions.

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 311c6018d503..b3cb3a17037b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -955,12 +955,14 @@ void free_compound_page(struct page *page);
> * pte_mkwrite. But get_user_pages can cause write faults for mappings
> * that do not have writing enabled, when used by access_process_vm.
> */
> +#ifndef maybe_mkwrite

maybe_mkwrite is defined in asm/pgtable.h. Where is the #include?

> static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> {
> if (likely(vma->vm_flags & VM_WRITE))
> pte = pte_mkwrite(pte);
> return pte;
> }
> +#endif
>
> vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page);
> void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 406a3c28c026..2adedcfca00b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -491,12 +491,14 @@ static int __init setup_transparent_hugepage(char *str)
> }
> __setup("transparent_hugepage=", setup_transparent_hugepage);
>
> +#ifndef maybe_pmd_mkwrite
> pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
> {
> if (likely(vma->vm_flags & VM_WRITE))
> pmd = pmd_mkwrite(pmd);
> return pmd;
> }
> +#endif
>
> #ifdef CONFIG_MEMCG
> static inline struct deferred_split *get_deferred_split_queue(struct page *page)