linux-kernel - Re: [PATCH 17/35] mm: Fixup places that call pte

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Message-ID: <8065c333-0911-04a2-f91e-7c2e0cc7ec51@intel.com>
Date:   Wed, 9 Feb 2022 13:51:42 -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 17/35] mm: Fixup places that call pte_mkwrite() directly

On 1/30/22 13:18, Rick Edgecombe wrote:
> - do_anonymous_page() and migrate_vma_insert_page() check VM_WRITE directly
>   and call pte_mkwrite(), which is the same as maybe_mkwrite().  Change
>   them to maybe_mkwrite().

Those look OK.

> - In do_numa_page(), if the numa entry was writable, then pte_mkwrite()
>   is called directly.  Fix it by doing maybe_mkwrite().  Make the same
>   changes to do_huge_pmd_numa_page().

This is another "what", not "why" changelog.  This change puzzles me.

*Why* is this needed?  It sounds like pte_mkwrite() doesn't work for
shadow stack PTEs.  Let's say that explicitly.

I also this this is ab/misuse of maybe_mkwrite().

The shadow stack VMA *REQUIRES* PTEs with Dirty=1.  There's no *maybe*
about it.  The rest of this is essentially a hack to get
VM_SHADOW_STACK-required bits into the PTE.  We have a place where we
store those VMA-required bits: vma->vm_page_prot.  Look at how we store
the pkey bits in there for instance.

Let's say we set _PAGE_DIRTY in vma->vm_page_prot.  We'd come into
do_anonymous_page() for instance and do this:

>         entry = mk_pte(page, vma->vm_page_prot); <--- PTE is Write=0,Dirty=1 Yay!
>         entry = pte_sw_mkyoung(entry);
>         if (vma->vm_flags & VM_WRITE) <--- False, skip the pte_mkwrite()
>                 entry = pte_mkwrite(pte_mkdirty(entry));

In other words, it "just works" because shadow stack VMAs don't have
VM_WRITE set.

I think the other VM_WRITE checks would be fine too, although I'm unsure
about the change_page_attr() one.