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Date:   Fri, 9 Apr 2021 18:31:46 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
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        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
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        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
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        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v24 19/30] mm: Update can_follow_write_pte() for shadow
 stack
Message-ID: <20210409153146.ib2xsn7n2q4ixpde@box.shutemov.name>
References: <20210401221104.31584-1-yu-cheng.yu@intel.com>
 <20210401221104.31584-20-yu-cheng.yu@intel.com>
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In-Reply-To: <20210401221104.31584-20-yu-cheng.yu@intel.com>
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On Thu, Apr 01, 2021 at 03:10:53PM -0700, Yu-cheng Yu wrote:
> 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, 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().
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> v24:
> - Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().
> 
>  mm/gup.c         | 8 +++++---
>  mm/huge_memory.c | 8 +++++---
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index e40579624f10..c313cc988865 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -356,10 +356,12 @@ 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));
> +		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte) &&
> +				  !is_shadow_stack_mapping(vma->vm_flags));

It's getting too ugly. I think it deserve to be rewritten. What about:

	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;
	return true;

?

-- 
 Kirill A. Shutemov