x86, kfence: enable KFENCE for x86

Add architecture specific implementation details for KFENCE and enable
KFENCE for the x86 architecture. In particular, this implements the
required interface in <asm/kfence.h> for setting up the pool and
providing helper functions for protecting and unprotecting pages.

For x86, we need to ensure that the pool uses 4K pages, which is done
using the set_memory_4k() helper function.

[[email protected]: add missing copyright and description header]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Marco Elver <[email protected]>
Signed-off-by: Alexander Potapenko <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>
Co-developed-by: Marco Elver <[email protected]>
Reviewed-by: Jann Horn <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christopher Lameter <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joern Engel <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/x86/Kconfig

@@ -151,6 +151,7 @@ config X86
 	select HAVE_ARCH_JUMP_LABEL_RELATIVE
 	select HAVE_ARCH_KASAN			if X86_64
 	select HAVE_ARCH_KASAN_VMALLOC		if X86_64
+	select HAVE_ARCH_KFENCE
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS		if MMU
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT

arch/x86/include/asm/kfence.h

@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * x86 KFENCE support.
+ *
+ * Copyright (C) 2020, Google LLC.
+ */
+
+#ifndef _ASM_X86_KFENCE_H
+#define _ASM_X86_KFENCE_H
+
+#include <linux/bug.h>
+#include <linux/kfence.h>
+
+#include <asm/pgalloc.h>
+#include <asm/pgtable.h>
+#include <asm/set_memory.h>
+#include <asm/tlbflush.h>
+
+/*
+ * The page fault handler entry function, up to which the stack trace is
+ * truncated in reports.
+ */
+#define KFENCE_SKIP_ARCH_FAULT_HANDLER "asm_exc_page_fault"
+
+/* Force 4K pages for __kfence_pool. */
+static inline bool arch_kfence_init_pool(void)
+{
+	unsigned long addr;
+
+	for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
+	     addr += PAGE_SIZE) {
+		unsigned int level;
+
+		if (!lookup_address(addr, &level))
+			return false;
+
+		if (level != PG_LEVEL_4K)
+			set_memory_4k(addr, 1);
+	}
+
+	return true;
+}
+
+/* Protect the given page and flush TLB. */
+static inline bool kfence_protect_page(unsigned long addr, bool protect)
+{
+	unsigned int level;
+	pte_t *pte = lookup_address(addr, &level);
+
+	if (WARN_ON(!pte || level != PG_LEVEL_4K))
+		return false;
+
+	/*
+	 * We need to avoid IPIs, as we may get KFENCE allocations or faults
+	 * with interrupts disabled. Therefore, the below is best-effort, and
+	 * does not flush TLBs on all CPUs. We can tolerate some inaccuracy;
+	 * lazy fault handling takes care of faults after the page is PRESENT.
+	 */
+
+	if (protect)
+		set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
+	else
+		set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
+
+	/* Flush this CPU's TLB. */
+	flush_tlb_one_kernel(addr);
+	return true;
+}
+
+#endif /* _ASM_X86_KFENCE_H */

arch/x86/mm/fault.c

@@ -9,6 +9,7 @@
 #include <linux/kdebug.h>		/* oops_begin/end, ...		*/
 #include <linux/extable.h>		/* search_exception_tables	*/
 #include <linux/memblock.h>		/* max_low_pfn			*/
+#include <linux/kfence.h>		/* kfence_handle_page_fault	*/
 #include <linux/kprobes.h>		/* NOKPROBE_SYMBOL, ...		*/
 #include <linux/mmiotrace.h>		/* kmmio_handler, ...		*/
 #include <linux/perf_event.h>		/* perf_sw_event		*/
@@ -680,6 +681,10 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
 	if (IS_ENABLED(CONFIG_EFI))
 		efi_crash_gracefully_on_page_fault(address);
 
+	/* Only not-present faults should be handled by KFENCE. */
+	if (!(error_code & X86_PF_PROT) && kfence_handle_page_fault(address))
+		return;
+
 oops:
 	/*
 	 * Oops. The kernel tried to access some bad page. We'll have to