linux-kernel - [PATCH v7 13/32] riscv mmu: write protect and shadow stack

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Message-Id: <20241029-v5_user_cfi_series-v7-13-2727ce9936cb@rivosinc.com>
Date: Tue, 29 Oct 2024 16:44:13 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
 Andrew Morton <akpm@...ux-foundation.org>, 
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
 Vlastimil Babka <vbabka@...e.cz>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
 Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>, 
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
 Christian Brauner <brauner@...nel.org>, 
 Peter Zijlstra <peterz@...radead.org>, Oleg Nesterov <oleg@...hat.com>, 
 Eric Biederman <ebiederm@...ssion.com>, Kees Cook <kees@...nel.org>, 
 Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 linux-mm@...ck.org, linux-riscv@...ts.infradead.org, 
 devicetree@...r.kernel.org, linux-arch@...r.kernel.org, 
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 alistair.francis@....com, richard.henderson@...aro.org, jim.shu@...ive.com, 
 andybnac@...il.com, kito.cheng@...ive.com, charlie@...osinc.com, 
 atishp@...osinc.com, evan@...osinc.com, cleger@...osinc.com, 
 alexghiti@...osinc.com, samitolvanen@...gle.com, broonie@...nel.org, 
 rick.p.edgecombe@...el.com, Deepak Gupta <debug@...osinc.com>
Subject: [PATCH v7 13/32] 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 shadow stack 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>
Alexandre Ghiti <alexghiti@...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 7963ab11d924..fdab7d74437d 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -411,7 +411,7 @@ static inline int pte_devmap(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) */
@@ -612,7 +612,15 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 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);
+	pte_t read_pte = READ_ONCE(*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

-- 
2.34.1