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Date: Wed, 9 Feb 2022 13:16:49 -0800 From: Dave Hansen <dave.hansen@...el.com> To: Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-arch@...r.kernel.org, linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>, Balbir Singh <bsingharora@...il.com>, Borislav Petkov <bp@...en8.de>, Cyrill Gorcunov <gorcunov@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Eugene Syromiatnikov <esyr@...hat.com>, Florian Weimer <fweimer@...hat.com>, "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Mike Kravetz <mike.kravetz@...cle.com>, Nadav Amit <nadav.amit@...il.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>, Peter Zijlstra <peterz@...radead.org>, Randy Dunlap <rdunlap@...radead.org>, "Ravi V . Shankar" <ravi.v.shankar@...el.com>, Dave Martin <Dave.Martin@....com>, Weijiang Yang <weijiang.yang@...el.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, joao.moreira@...el.com, John Allen <john.allen@....com>, kcc@...gle.com, eranian@...gle.com Cc: Yu-cheng Yu <yu-cheng.yu@...el.com> Subject: Re: [PATCH 16/35] x86/mm: Update maybe_mkwrite() for shadow stack 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)
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