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From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        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@chromiun.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: [RFC PATCH v2 13/27] mm: Handle shadow stack page fault
Date:   Tue, 10 Jul 2018 15:26:25 -0700
Message-Id: <20180710222639.8241-14-yu-cheng.yu@intel.com>
In-Reply-To: <20180710222639.8241-1-yu-cheng.yu@intel.com>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When a task does fork(), its shadow stack must be duplicated for
the child.  However, the child may not actually use all pages of
of the copied shadow stack.  This patch implements a flow that
is similar to copy-on-write of an anonymous page, but for shadow
stack memory.  A shadow stack PTE needs to be RO and dirty.  We
use this dirty bit requirement to effect the copying of shadow
stack pages.

In copy_one_pte(), we clear the dirty bit from the shadow stack
PTE.  On the next shadow stack access to the PTE, a page fault
occurs.  At that time, we then copy/re-use the page and fix the
PTE.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
 mm/memory.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7206a634270b..a2695dbc0418 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2453,7 +2453,13 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
 
 	flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
 	entry = pte_mkyoung(vmf->orig_pte);
-	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+
+	if (is_shstk_mapping(vma->vm_flags))
+		entry = pte_mkdirty_shstk(entry);
+	else
+		entry = pte_mkdirty(entry);
+
+	entry = maybe_mkwrite(entry, vma);
 	if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
 		update_mmu_cache(vma, vmf->address, vmf->pte);
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -2526,7 +2532,11 @@ static int wp_page_copy(struct vm_fault *vmf)
 		}
 		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
 		entry = mk_pte(new_page, vma->vm_page_prot);
-		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+		if (is_shstk_mapping(vma->vm_flags))
+			entry = pte_mkdirty_shstk(entry);
+		else
+			entry = pte_mkdirty(entry);
+		entry = maybe_mkwrite(entry, vma);
 		/*
 		 * Clear the pte entry and flush it first, before updating the
 		 * pte with the new entry. This will avoid a race condition
@@ -3201,6 +3211,14 @@ static int do_anonymous_page(struct vm_fault *vmf)
 	mem_cgroup_commit_charge(page, memcg, false, false);
 	lru_cache_add_active_or_unevictable(page, vma);
 setpte:
+	/*
+	 * If this is within a shadow stack mapping, mark
+	 * the PTE dirty.  We don't use pte_mkdirty(),
+	 * because the PTE must have _PAGE_DIRTY_HW set.
+	 */
+	if (is_shstk_mapping(vma->vm_flags))
+		entry = pte_mkdirty_shstk(entry);
+
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
 
 	/* No need to invalidate - it was non-present before */
@@ -3983,6 +4001,14 @@ static int handle_pte_fault(struct vm_fault *vmf)
 	entry = vmf->orig_pte;
 	if (unlikely(!pte_same(*vmf->pte, entry)))
 		goto unlock;
+
+	/*
+	 * Shadow stack PTEs are copy-on-access, so do_wp_page()
+	 * handling on them no matter if we have write fault or not.
+	 */
+	if (is_shstk_mapping(vmf->vma->vm_flags))
+		return do_wp_page(vmf);
+
 	if (vmf->flags & FAULT_FLAG_WRITE) {
 		if (!pte_write(entry))
 			return do_wp_page(vmf);
-- 
2.17.1