linux-kernel - Re: [PATCH 20/35] mm: Update can_follow_write

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Message-ID: <74038286-6ff3-7eb2-ea65-2e223a894900@intel.com>
Date:   Wed, 9 Feb 2022 14:50:27 -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 20/35] mm: Update can_follow_write_pte() for shadow stack

On 1/30/22 13:18, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> Can_follow_write_pte() ensures a read-only page is COWed by checking the
> FOLL_COW flag, and uses pte_dirty() to validate the flag is still valid.
> 
> Like a writable data page, a shadow stack page is writable, and becomes
> read-only during copy-on-write,

I thought we could not have read-only shadow stack pages.  What does a
read-only shadow stack PTE look like? ;)

> but it is always dirty.  Thus, in the
> can_follow_write_pte() check, it belongs to the writable page case and
> should be excluded from the read-only page pte_dirty() check.  Apply
> the same changes to can_follow_write_pmd().
> 
> While at it, also split the long line into smaller ones.

FWIW, I probably would have had a preparatory patch for this part.  The
advantage is that if you break existing code, it's a lot easier to
figure it out if you have a separate refactoring patch.  Also, for a
patch like this, the refactoring might result in the same exact binary.
 It's a pretty good sign that your patch won't cause regressions if it
results in the same binary.

> diff --git a/mm/gup.c b/mm/gup.c
> index f0af462ac1e2..95b7d1084c44 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -464,10 +464,18 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
>   * FOLL_FORCE can write to even unwritable pte's, but only
>   * after we've gone through a COW cycle and they are dirty.
>   */
> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
> +					struct vm_area_struct *vma)
>  {
> -	return pte_write(pte) ||
> -		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> +	if (pte_write(pte))
> +		return true;
> +	if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
> +		return false;
> +	if (!pte_dirty(pte))
> +		return false;
> +	if (is_shadow_stack_mapping(vma->vm_flags))
> +		return false;

You had me up until this is_shadow_stack_mapping().  It wasn't mentioned
at all in the changelog.  Logically, I think it's trying to say that a
shadow stack VMA never allows a FOLL_FORCE override.

That makes some sense, but it's a pretty big point not to mention in the
changelog.

> +	return true;
>  }
>  
>  static struct page *follow_page_pte(struct vm_area_struct *vma,
> @@ -510,7 +518,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  	}
>  	if ((flags & FOLL_NUMA) && pte_protnone(pte))
>  		goto no_page;
> -	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
> +	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags, vma)) {
>  		pte_unmap_unlock(ptep, ptl);
>  		return NULL;
>  	}