UML: add support for KASAN under x86_64

Make KASAN run on User Mode Linux on x86_64.

The UML-specific KASAN initializer uses mmap to map the ~16TB of shadow
memory to the location defined by KASAN_SHADOW_OFFSET. kasan_init()
utilizes constructors to initialize KASAN before main().

The location of the KASAN shadow memory, starting at
KASAN_SHADOW_OFFSET, can be configured using the KASAN_SHADOW_OFFSET
option. The default location of this offset is 0x100000000000, which
keeps it out-of-the-way even on UML setups with more "physical" memory.

For low-memory setups, 0x7fff8000 can be used instead, which fits in an
immediate and is therefore faster, as suggested by Dmitry Vyukov. There
is usually enough free space at this location; however, it is a config
option so that it can be easily changed if needed.

Note that, unlike KASAN on other architectures, vmalloc allocations
still use the shadow memory allocated upfront, rather than allocating
and free-ing it per-vmalloc allocation.

If another architecture chooses to go down the same path, we should
replace the checks for CONFIG_UML with something more generic, such
as:
- A CONFIG_KASAN_NO_SHADOW_ALLOC option, which architectures could set
- or, a way of having architecture-specific versions of these vmalloc
and module shadow memory allocation options.

Also note that, while UML supports both KASAN in inline mode
(CONFIG_KASAN_INLINE) and static linking (CONFIG_STATIC_LINK), it does
not support both at the same time.

Signed-off-by: Patricia Alfonso <[e-mail]>
Co-developed-by: Vincent Whitchurch <[e-mail]>
Signed-off-by: Vincent Whitchurch <[e-mail]>
Signed-off-by: David Gow <[e-mail]>
Reviewed-by: Johannes Berg <[e-mail]>
Reviewed-by: Dmitry Vyukov <[e-mail]>
Reviewed-by: Andrey Konovalov <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/Kconfig

···
 	select ARCH_HAS_STRNLEN_USER
 	select ARCH_NO_PREEMPT
 	select HAVE_ARCH_AUDITSYSCALL
+	select HAVE_ARCH_KASAN if X86_64
+	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_UID16
···
 	  line help for more details.
 
 	  It is safe to say Y, but you probably don't need this.
+
+config KASAN_SHADOW_OFFSET
+	hex
+	depends on KASAN
+	default 0x100000000000
+	help
+	  This is the offset at which the ~16TB of shadow memory is
+	  mapped and used by KASAN for memory debugging. This can be any
+	  address that has at least KASAN_SHADOW_SIZE (total address space divided
+	  by 8) amount of space so that the KASAN shadow memory does not conflict
+	  with anything. The default is 0x100000000000, which works even if mem is
+	  set to a large value. On low-memory systems, try 0x7fff8000, as it fits
+	  into the immediate of most instructions, improving performance.
 
 endmenu
 

arch/um/include/asm/common.lds.S

···
   }
   .init_array : {
 	__init_array_start = .;
+	*(.kasan_init)
+	*(.init_array.*)
 	*(.init_array)
 	__init_array_end = .;
   }

arch/um/include/asm/kasan.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_UM_KASAN_H
+#define __ASM_UM_KASAN_H
+
+#include <linux/init.h>
+#include <linux/const.h>
+
+#define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
+
+/* used in kasan_mem_to_shadow to divide by 8 */
+#define KASAN_SHADOW_SCALE_SHIFT 3
+
+#ifdef CONFIG_X86_64
+#define KASAN_HOST_USER_SPACE_END_ADDR 0x00007fffffffffffUL
+/* KASAN_SHADOW_SIZE is the size of total address space divided by 8 */
+#define KASAN_SHADOW_SIZE ((KASAN_HOST_USER_SPACE_END_ADDR + 1) >> \
+			KASAN_SHADOW_SCALE_SHIFT)
+#else
+#error "KASAN_SHADOW_SIZE is not defined for this sub-architecture"
+#endif /* CONFIG_X86_64 */
+
+#define KASAN_SHADOW_START (KASAN_SHADOW_OFFSET)
+#define KASAN_SHADOW_END (KASAN_SHADOW_START + KASAN_SHADOW_SIZE)
+
+#ifdef CONFIG_KASAN
+void kasan_init(void);
+void kasan_map_memory(void *start, unsigned long len);
+extern int kasan_um_is_ready;
+
+#ifdef CONFIG_STATIC_LINK
+#define kasan_arch_is_ready() (kasan_um_is_ready)
+#endif
+#else
+static inline void kasan_init(void) { }
+#endif /* CONFIG_KASAN */
+
+#endif /* __ASM_UM_KASAN_H */

arch/um/kernel/dyn.lds.S

···
      be empty, which isn't pretty.  */
   . = ALIGN(32 / 8);
   .preinit_array     : { *(.preinit_array) }
-  .init_array     : { *(.init_array) }
+  .init_array     : {
+    *(.kasan_init)
+    *(.init_array.*)
+    *(.init_array)
+  }
   .fini_array     : { *(.fini_array) }
   .data           : {
     INIT_TASK_DATA(KERNEL_STACK_SIZE)

arch/um/kernel/mem.c

···
 #include <kern_util.h>
 #include <mem_user.h>
 #include <os.h>
+#include <linux/sched/task.h>
+
+#ifdef CONFIG_KASAN
+int kasan_um_is_ready;
+void kasan_init(void)
+{
+	/*
+	 * kasan_map_memory will map all of the required address space and
+	 * the host machine will allocate physical memory as necessary.
+	 */
+	kasan_map_memory((void *)KASAN_SHADOW_START, KASAN_SHADOW_SIZE);
+	init_task.kasan_depth = 0;
+	kasan_um_is_ready = true;
+}
+
+static void (*kasan_init_ptr)(void)
+__section(".kasan_init") __used
+= kasan_init;
+#endif
 
 /* allocated in paging_init, zeroed in mem_init, and unchanged thereafter */
 unsigned long *empty_zero_page = NULL;

arch/um/kernel/stacktrace.c

···
 
 	frame = (struct stack_frame *)bp;
 	while (((long) sp & (THREAD_SIZE-1)) != 0) {
-		addr = *sp;
+		addr = READ_ONCE_NOCHECK(*sp);
 		if (__kernel_text_address(addr)) {
 			reliable = 0;
 			if ((unsigned long) sp == bp + sizeof(long)) {

arch/um/os-Linux/mem.c

···
 #include <init.h>
 #include <os.h>
 
+/*
+ * kasan_map_memory - maps memory from @start with a size of @len.
+ * The allocated memory is filled with zeroes upon success.
+ * @start: the start address of the memory to be mapped
+ * @len: the length of the memory to be mapped
+ *
+ * This function is used to map shadow memory for KASAN in uml
+ */
+void kasan_map_memory(void *start, size_t len)
+{
+	if (mmap(start,
+		 len,
+		 PROT_READ|PROT_WRITE,
+		 MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE,
+		 -1,
+		 0) == MAP_FAILED) {
+		os_info("Couldn't allocate shadow memory: %s\n.",
+			strerror(errno));
+		exit(1);
+	}
+}
+
 /* Set by make_tempfile() during early boot. */
 static char *tempdir = NULL;
 

arch/um/os-Linux/user_syms.c

···
 #ifndef __x86_64__
 extern void *memcpy(void *, const void *, size_t);
 EXPORT_SYMBOL(memcpy);
-#endif
-
 EXPORT_SYMBOL(memmove);
 EXPORT_SYMBOL(memset);
+#endif
+
 EXPORT_SYMBOL(printf);
 
 /* Here, instead, I can provide a fake prototype. Yes, someone cares: genksyms.

arch/x86/um/Makefile

···
 
 obj-y += syscalls_64.o vdso/
 
-subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o
+subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o \
+	../lib/memmove_64.o ../lib/memset_64.o
 
 endif
 

arch/x86/um/vdso/Makefile

···
 # Building vDSO images for x86.
 #
 
+# do not instrument on vdso because KASAN is not compatible with user mode
+KASAN_SANITIZE			:= n
+
 # Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
 KCOV_INSTRUMENT                := n
 

mm/kasan/shadow.c

···
 		return 0;
 
 	shadow_start = (unsigned long)kasan_mem_to_shadow((void *)addr);
-	shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
 	shadow_end = (unsigned long)kasan_mem_to_shadow((void *)addr + size);
-	shadow_end = ALIGN(shadow_end, PAGE_SIZE);
+
+	/*
+	 * User Mode Linux maps enough shadow memory for all of virtual memory
+	 * at boot, so doesn't need to allocate more on vmalloc, just clear it.
+	 *
+	 * The remaining CONFIG_UML checks in this file exist for the same
+	 * reason.
+	 */
+	if (IS_ENABLED(CONFIG_UML)) {
+		__memset((void *)shadow_start, KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
+		return 0;
+	}
+
+	shadow_start = PAGE_ALIGN_DOWN(shadow_start);
+	shadow_end = PAGE_ALIGN(shadow_end);
 
 	ret = apply_to_page_range(&init_mm, shadow_start,
 				  shadow_end - shadow_start,
···
 
 	if (shadow_end > shadow_start) {
 		size = shadow_end - shadow_start;
+		if (IS_ENABLED(CONFIG_UML)) {
+			__memset(shadow_start, KASAN_SHADOW_INIT, shadow_end - shadow_start);
+			return;
+		}
 		apply_to_existing_page_range(&init_mm,
 					     (unsigned long)shadow_start,
 					     size, kasan_depopulate_vmalloc_pte,
···
 	if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
 		return -EINVAL;
 
+	if (IS_ENABLED(CONFIG_UML)) {
+		__memset((void *)shadow_start, KASAN_SHADOW_INIT, shadow_size);
+		return 0;
+	}
+
 	ret = __vmalloc_node_range(shadow_size, 1, shadow_start,
 			shadow_start + shadow_size,
 			GFP_KERNEL,
···
 
 void kasan_free_module_shadow(const struct vm_struct *vm)
 {
+	if (IS_ENABLED(CONFIG_UML))
+		return;
+
 	if (vm->flags & VM_KASAN)
 		vfree(kasan_mem_to_shadow(vm->addr));
 }