linux-kernel - [PATCH v1 RFC Zisslpcfi 10/20] riscv mmu: write protect and shadow stack

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Date:   Sun, 12 Feb 2023 20:53:39 -0800
From:   Deepak Gupta <debug@...osinc.com>
To:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     Deepak Gupta <debug@...osinc.com>
Subject: [PATCH v1 RFC Zisslpcfi 10/20] riscv mmu: write protect and shadow stack

`fork` implements copy on write (COW) by making pages readonly in child
and parent both.

ptep_set_wrprotect and pte_wrprotect clears _PAGE_WRITE in PTE.
Assumption is that page is readable and on fault copy on write happens.

To implement COW on such pages, clearing up W bit makes them XWR = 000.
This will result in wrong PTE setting which says no perms but V=1 and PFN
field pointing to final page. Instead desired behavior is to turn it into
a readable page, take an access (load/store) fault on sspush/sspop
(shadow stack) and then perform COW on such pages. This way regular reads
would still be allowed and not lead to COW maintaining current behavior
of COW on non-shadow stack but writeable memory.

On the other hand it doesn't interfere with existing COW for read-write
memory. Assumption is always that _PAGE_READ must have been set and thus
setting _PAGE_READ is harmless.

Signed-off-by: Deepak Gupta <debug@...osinc.com>
---
 arch/riscv/include/asm/pgtable.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 74dbe122f2fa..13b325253c99 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -334,7 +334,7 @@ static inline int pte_special(pte_t pte)

 static inline pte_t pte_wrprotect(pte_t pte)
 {
-	return __pte(pte_val(pte) & ~(_PAGE_WRITE));
+	return __pte((pte_val(pte) & ~(_PAGE_WRITE)) | (_PAGE_READ));
 }

 /* static inline pte_t pte_mkread(pte_t pte) */
@@ -509,7 +509,15 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
 static inline void ptep_set_wrprotect(struct mm_struct *mm,
 				      unsigned long address, pte_t *ptep)
 {
-	atomic_long_and(~(unsigned long)_PAGE_WRITE, (atomic_long_t *)ptep);
+	volatile pte_t read_pte = *ptep;
+	/*
+	 * ptep_set_wrprotect can be called for shadow stack ranges too.
+	 * shadow stack memory is XWR = 010 and thus clearing _PAGE_WRITE will lead to
+	 * encoding 000b which is wrong encoding with V = 1. This should lead to page fault
+	 * but we dont want this wrong configuration to be set in page tables.
+	 */
+	atomic_long_set((atomic_long_t *)ptep,
+			((pte_val(read_pte) & ~(unsigned long)_PAGE_WRITE) | _PAGE_READ));
 }

 #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
-- 
2.25.1