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Date:   Thu, 22 Jul 2021 13:51:59 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH v28 12/32] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for
shadow stack.  Copy-on-write PTEs have [R/O + _PAGE_COW].

When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could
become a transient shadow stack PTE in two cases:

The first case is that some processors can start a write but end up seeing
a read-only PTE by the time they get to the Dirty bit, creating a transient
shadow stack PTE.  However, this will not occur on processors supporting
Shadow Stack, and a TLB flush is not necessary.

The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non-
atomically, a transient shadow stack PTE can be created as a result.
Thus, prevent that with cmpxchg.

Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many
insights to the issue.  Jann Horn provided the cmpxchg solution.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 arch/x86/include/asm/pgtable.h | 36 ++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index cf7316e968df..df4ce715560a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1278,6 +1278,24 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
 static inline void ptep_set_wrprotect(struct mm_struct *mm,
 				      unsigned long addr, pte_t *ptep)
 {
+	/*
+	 * If Shadow Stack is enabled, pte_wrprotect() moves _PAGE_DIRTY
+	 * to _PAGE_COW (see comments at pte_wrprotect()).
+	 * When a thread reads a RW=1, Dirty=0 PTE and before changing it
+	 * to RW=0, Dirty=0, another thread could have written to the page
+	 * and the PTE is RW=1, Dirty=1 now.  Use try_cmpxchg() to detect
+	 * PTE changes and update old_pte, then try again.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		pte_t old_pte, new_pte;
+
+		old_pte = READ_ONCE(*ptep);
+		do {
+			new_pte = pte_wrprotect(old_pte);
+		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
+
+		return;
+	}
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
 }
 
@@ -1322,6 +1340,24 @@ static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm,
 static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 				      unsigned long addr, pmd_t *pmdp)
 {
+	/*
+	 * If Shadow Stack is enabled, pmd_wrprotect() moves _PAGE_DIRTY
+	 * to _PAGE_COW (see comments at pmd_wrprotect()).
+	 * When a thread reads a RW=1, Dirty=0 PMD and before changing it
+	 * to RW=0, Dirty=0, another thread could have written to the page
+	 * and the PMD is RW=1, Dirty=1 now.  Use try_cmpxchg() to detect
+	 * PMD changes and update old_pmd, then try again.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		pmd_t old_pmd, new_pmd;
+
+		old_pmd = READ_ONCE(*pmdp);
+		do {
+			new_pmd = pmd_wrprotect(old_pmd);
+		} while (!try_cmpxchg((pmdval_t *)pmdp, (pmdval_t *)&old_pmd, pmd_val(new_pmd)));
+
+		return;
+	}
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
 }
 
-- 
2.21.0

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