Re: [PATCH 16/35] x86/mm: Update maybe_mkwrite() for shadow stack
From: Dave Hansen <hidden>
Date: 2022-02-09 21:17:06
Also in:
linux-arch, linux-doc, linux-mm, lkml
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?
quoted hunk ↗ jump to hunk
#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.
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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?
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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)