[RFC] riscv: Enable KFENCE for riscv64

[Liu Shixin]

Add architecture specific implementation details for KFENCE and enable
KFENCE for the riscv64 architecture. In particular, this implements the
required interface in <asm/kfence.h>.

KFENCE requires that attributes for pages from its memory pool can
individually be set. Therefore, force the kfence pool to be mapped at
page granularity.

I tested this patch using the testcases in kfence_test.c and all passed.

Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
 arch/riscv/Kconfig              |  1 +
 arch/riscv/include/asm/kfence.h | 51 +++++++++++++++++++++++++++++++++
 arch/riscv/mm/fault.c           | 11 ++++++-
 3 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/kfence.h

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c426e7d20907..000d8aba1030 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -64,6 +64,7 @@ config RISCV
 	select HAVE_ARCH_JUMP_LABEL_RELATIVE
 	select HAVE_ARCH_KASAN if MMU && 64BIT
 	select HAVE_ARCH_KASAN_VMALLOC if MMU && 64BIT
+	select HAVE_ARCH_KFENCE if MMU && 64BIT
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_KGDB_QXFER_PKT
 	select HAVE_ARCH_MMAP_RND_BITS if MMU
diff --git a/arch/riscv/include/asm/kfence.h b/arch/riscv/include/asm/kfence.h
new file mode 100644
index 000000000000..590c5b7e3514
--- /dev/null
+++ b/arch/riscv/include/asm/kfence.h
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ASM_RISCV_KFENCE_H
+#define _ASM_RISCV_KFENCE_H
+
+#include <linux/pfn.h>
+#include <linux/slab.h>
+#include <linux/kfence.h>
+#include <asm/pgtable.h>
+
+static inline bool arch_kfence_init_pool(void)
+{
+	int i;
+	unsigned long addr;
+	pte_t *pte;
+	pmd_t *pmd;
+
+	for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
+	     addr += PAGE_SIZE) {
+		pte = virt_to_kpte(addr);
+		pmd = pmd_off_k(addr);
+
+		if (!pmd_leaf(*pmd) && pte_present(*pte))
+			continue;
+
+		pte = kmalloc(PAGE_SIZE, GFP_ATOMIC);
+		for (i = 0; i < PTRS_PER_PTE; i++)
+			set_pte(pte + i, pfn_pte(PFN_DOWN(__pa((addr & PMD_MASK) + i * PAGE_SIZE)), PAGE_KERNEL));
+
+		set_pmd(pmd, pfn_pmd(PFN_DOWN(__pa(pte)), PAGE_TABLE));
+		flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+	}
+
+	return true;
+}
+
+static inline bool kfence_protect_page(unsigned long addr, bool protect)
+{
+	pte_t *pte = virt_to_kpte(addr);
+
+	if (protect)
+		set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
+	else
+		set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
+
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	return true;
+}
+
+#endif /* _ASM_RISCV_KFENCE_H */
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 096463cc6fff..aa08dd2f8fae 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -14,6 +14,7 @@
 #include <linux/signal.h>
 #include <linux/uaccess.h>
 #include <linux/kprobes.h>
+#include <linux/kfence.h>
 
 #include <asm/ptrace.h>
 #include <asm/tlbflush.h>
@@ -45,7 +46,15 @@ static inline void no_context(struct pt_regs *regs, unsigned long addr)
 	 * Oops. The kernel tried to access some bad page. We'll have to
 	 * terminate things with extreme prejudice.
 	 */
-	msg = (addr < PAGE_SIZE) ? "NULL pointer dereference" : "paging request";
+	if (addr < PAGE_SIZE)
+		msg = "NULL pointer dereference";
+	else {
+		if (kfence_handle_page_fault(addr, regs->cause == EXC_STORE_PAGE_FAULT, regs))
+			return;
+
+		msg = "paging request";
+	}
+
 	die_kernel_fault(msg, addr, regs);
 }
 
-- 
2.25.1