riscv: Enable KFENCE for riscv64

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.

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

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

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

arch/riscv/include/asm/kfence.h

@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ASM_RISCV_KFENCE_H
+#define _ASM_RISCV_KFENCE_H
+
+#include <linux/kfence.h>
+#include <linux/pfn.h>
+#include <asm-generic/pgalloc.h>
+#include <asm/pgtable.h>
+
+static inline int split_pmd_page(unsigned long addr)
+{
+	int i;
+	unsigned long pfn = PFN_DOWN(__pa((addr & PMD_MASK)));
+	pmd_t *pmd = pmd_off_k(addr);
+	pte_t *pte = pte_alloc_one_kernel(&init_mm);
+
+	if (!pte)
+		return -ENOMEM;
+
+	for (i = 0; i < PTRS_PER_PTE; i++)
+		set_pte(pte + i, pfn_pte(pfn + i, PAGE_KERNEL));
+	set_pmd(pmd, pfn_pmd(PFN_DOWN(__pa(pte)), PAGE_TABLE));
+
+	flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+	return 0;
+}
+
+static inline bool arch_kfence_init_pool(void)
+{
+	int ret;
+	unsigned long addr;
+	pmd_t *pmd;
+
+	for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
+	     addr += PAGE_SIZE) {
+		pmd = pmd_off_k(addr);
+
+		if (pmd_leaf(*pmd)) {
+			ret = split_pmd_page(addr);
+			if (ret)
+				return false;
+		}
+	}
+
+	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 */

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);
 }