linux-kernel - Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write

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Date:   Tue, 17 Jul 2018 16:03:30 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.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@...omiun.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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack

On Fri, 2018-07-13 at 11:26 -0700, Dave Hansen wrote:
> On 07/11/2018 10:05 AM, Yu-cheng Yu wrote:
> > 
> > My understanding is that we don't want to follow write pte if the page
> > is shared as read-only.  For a SHSTK page, that is (R/O + DIRTY_SW),
> > which means the SHSTK page has not been COW'ed.  Is that right?
> Let's look at the code again:
> 
> > 
> > -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,
> > +					bool shstk)
> >  {
> > +	bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte);
> > +
> >  	return pte_write(pte) ||
> > -		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> > +		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed);
> >  }
> This is another case where the naming of pte_*() is biting us vs. the
> perversion of the PTE bits.  The lack of comments and explanation inthe
> patch is compounding the confusion.
> 
> We need to find a way to differentiate "someone can write to this PTE"
> from "the write bit is set in this PTE".
> 
> In this particular hunk, we need to make it clear that pte_write() is
> *never* true for shadowstack PTEs.  In other words, shadow stack VMAs
> will (should?) never even *see* a pte_write() PTE.
> 
> I think this is a case where you just need to bite the bullet and
> bifurcate can_follow_write_pte().  Just separate the shadowstack and
> non-shadowstack parts.

In case I don't understand the exact issue.
What about the following.

diff --git a/mm/gup.c b/mm/gup.c
index fc5f98069f4e..45a0837b27f9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -70,6 +70,12 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
 }
 
+static inline bool can_follow_write_shstk_pte(pte_t pte, unsigned int flags)
+{
+	return ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
+		is_shstk_pte(pte));
+}
+
 static struct page *follow_page_pte(struct vm_area_struct *vma,
 		unsigned long address, pmd_t *pmd, unsigned int flags)
 {
@@ -105,9 +111,16 @@ 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)) {
-		pte_unmap_unlock(ptep, ptl);
-		return NULL;
+	if (flags & FOLL_WRITE) {
+		if (is_shstk_mapping(vma->vm_flags)) {
+			if (!can_follow_write_shstk_pte(pte, flags)) {
+				pte_unmap_unlock(ptep, ptl);
+				return NULL;
+			}
+		} else if (!can_follow_write_pte(pte, flags) {
+			pte_unmap_unlock(ptep, ptl);
+			return NULL;
+		}
 	}
 
 	page = vm_normal_page(vma, address, pte);