The Kernel Address Sanitizer (KASAN)

Overview

KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN (similar to userspace ASan) and software tag-based KASAN (similar to userspace HWASan).

KASAN uses compile-time instrumentation to insert validity checks before every memory access, and therefore requires a compiler version that supports that.

Generic KASAN is supported in both GCC and Clang. With GCC it requires version 4.9.2 or later for basic support and version 5.0 or later for detection of out-of-bounds accesses for stack and global variables and for inline instrumentation mode (see the Usage section). With Clang it requires version 7.0.0 or later and it doesn’t support detection of out-of-bounds accesses for global variables yet.

Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.

Currently generic KASAN is supported for the x86_64, arm64, xtensa, s390 and riscv architectures, and tag-based KASAN is supported only for arm64.

Usage

To enable KASAN configure kernel with:

CONFIG_KASAN = y

and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN).

You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types. The former produces smaller binary while the latter is 1.1 - 2 times faster.

Both KASAN modes work with both SLUB and SLAB memory allocators. For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.

To augment reports with last allocation and freeing stack of the physical page, it is recommended to enable also CONFIG_PAGE_OWNER and boot with page_owner=on.

To disable instrumentation for specific files or directories, add a line similar to the following to the respective kernel Makefile:

• For a single file (e.g. main.o):

  KASAN_SANITIZE_main.o := n
  

• For all files in one directory:

  KASAN_SANITIZE := n
  

Error reports

A typical out-of-bounds access generic KASAN report looks like this:

==================================================================
BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
Write of size 1 at addr ffff8801f44ec37b by task insmod/2760

CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
 dump_stack+0x94/0xd8
 print_address_description+0x73/0x280
 kasan_report+0x144/0x187
 __asan_report_store1_noabort+0x17/0x20
 kmalloc_oob_right+0xa8/0xbc [test_kasan]
 kmalloc_tests_init+0x16/0x700 [test_kasan]
 do_one_initcall+0xa5/0x3ae
 do_init_module+0x1b6/0x547
 load_module+0x75df/0x8070
 __do_sys_init_module+0x1c6/0x200
 __x64_sys_init_module+0x6e/0xb0
 do_syscall_64+0x9f/0x2c0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f96443109da
RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 2760:
 save_stack+0x43/0xd0
 kasan_kmalloc+0xa7/0xd0
 kmem_cache_alloc_trace+0xe1/0x1b0
 kmalloc_oob_right+0x56/0xbc [test_kasan]
 kmalloc_tests_init+0x16/0x700 [test_kasan]
 do_one_initcall+0xa5/0x3ae
 do_init_module+0x1b6/0x547
 load_module+0x75df/0x8070
 __do_sys_init_module+0x1c6/0x200
 __x64_sys_init_module+0x6e/0xb0
 do_syscall_64+0x9f/0x2c0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 815:
 save_stack+0x43/0xd0
 __kasan_slab_free+0x135/0x190
 kasan_slab_free+0xe/0x10
 kfree+0x93/0x1a0
 umh_complete+0x6a/0xa0
 call_usermodehelper_exec_async+0x4c3/0x640
 ret_from_fork+0x35/0x40

The buggy address belongs to the object at ffff8801f44ec300
 which belongs to the cache kmalloc-128 of size 128
The buggy address is located 123 bytes inside of
 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
The buggy address belongs to the page:
page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
flags: 0x200000000000100(slab)
raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
                                                                ^
 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
==================================================================

The header of the report provides a short summary of what kind of bug happened and what kind of access caused it. It’s followed by a stack trace of the bad access, a stack trace of where the accessed memory was allocated (in case bad access happens on a slab object), and a stack trace of where the object was freed (in case of a use-after-free bug report). Next comes a description of the accessed slab object and information about the accessed memory page.

In the last section the report shows memory state around the accessed address. Reading this part requires some understanding of how KASAN works.

The state of each 8 aligned bytes of memory is encoded in one shadow byte. Those 8 bytes can be accessible, partially accessible, freed or be a redzone. We use the following encoding for each shadow byte: 0 means that all 8 bytes of the corresponding memory region are accessible; number N (1 <= N <= 7) means that the first N bytes are accessible, and other (8 - N) bytes are not; any negative value indicates that the entire 8-byte word is inaccessible. We use different negative values to distinguish between different kinds of inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).

In the report above the arrows point to the shadow byte 03, which means that the accessed address is partially accessible.

For tag-based KASAN this last report section shows the memory tags around the accessed address (see Implementation details section).

Implementation details

Generic KASAN

From a high level, our approach to memory error detection is similar to that of kmemcheck: use shadow memory to record whether each byte of memory is safe to access, and use compile-time instrumentation to insert checks of shadow memory on each memory access.

Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and offset to translate a memory address to its corresponding shadow address.

Here is the function which translates an address to its corresponding shadow address:

static inline void *kasan_mem_to_shadow(const void *addr)
{
    return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
            + KASAN_SHADOW_OFFSET;
}

where KASAN_SHADOW_SCALE_SHIFT = 3.

Compile-time instrumentation is used to insert memory access checks. Compiler inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each memory access of size 1, 2, 4, 8 or 16. These functions check whether memory access is valid or not by checking corresponding shadow memory.

GCC 5.0 has possibility to perform inline instrumentation. Instead of making function calls GCC directly inserts the code to check the shadow memory. This option significantly enlarges kernel but it gives x1.1-x2 performance boost over outline instrumented kernel.

Software tag-based KASAN

Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to store a pointer tag in the top byte of kernel pointers. Like generic KASAN it uses shadow memory to store memory tags associated with each 16-byte memory cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).

On each memory allocation tag-based KASAN generates a random tag, tags the allocated memory with this tag, and embeds this tag into the returned pointer. Software tag-based KASAN uses compile-time instrumentation to insert checks before each memory access. These checks make sure that tag of the memory that is being accessed is equal to tag of the pointer that is used to access this memory. In case of a tag mismatch tag-based KASAN prints a bug report.

Software tag-based KASAN also has two instrumentation modes (outline, that emits callbacks to check memory accesses; and inline, that performs the shadow memory checks inline). With outline instrumentation mode, a bug report is simply printed from the function that performs the access check. With inline instrumentation a brk instruction is emitted by the compiler, and a dedicated brk handler is used to print bug reports.

A potential expansion of this mode is a hardware tag-based mode, which would use hardware memory tagging support instead of compiler instrumentation and manual shadow memory manipulation.

What memory accesses are sanitised by KASAN?

The kernel maps memory in a number of different parts of the address space. This poses something of a problem for KASAN, which requires that all addresses accessed by instrumented code have a valid shadow region.

The range of kernel virtual addresses is large: there is not enough real memory to support a real shadow region for every address that could be accessed by the kernel.

By default

By default, architectures only map real memory over the shadow region for the linear mapping (and potentially other small areas). For all other areas - such as vmalloc and vmemmap space - a single read-only page is mapped over the shadow area. This read-only shadow page declares all memory accesses as permitted.

This presents a problem for modules: they do not live in the linear mapping, but in a dedicated module space. By hooking in to the module allocator, KASAN can temporarily map real shadow memory to cover them. This allows detection of invalid accesses to module globals, for example.

This also creates an incompatibility with VMAP_STACK: if the stack lives in vmalloc space, it will be shadowed by the read-only page, and the kernel will fault when trying to set up the shadow data for stack variables.

CONFIG_KASAN_VMALLOC

With CONFIG_KASAN_VMALLOC, KASAN can cover vmalloc space at the cost of greater memory usage. Currently this is only supported on x86.

This works by hooking into vmalloc and vmap, and dynamically allocating real shadow memory to back the mappings.

Most mappings in vmalloc space are small, requiring less than a full page of shadow space. Allocating a full shadow page per mapping would therefore be wasteful. Furthermore, to ensure that different mappings use different shadow pages, mappings would have to be aligned to KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.

Instead, we share backing space across multiple mappings. We allocate a backing page when a mapping in vmalloc space uses a particular page of the shadow region. This page can be shared by other vmalloc mappings later on.

We hook in to the vmap infrastructure to lazily clean up unused shadow memory.

To avoid the difficulties around swapping mappings around, we expect that the part of the shadow region that covers the vmalloc space will not be covered by the early shadow page, but will be left unmapped. This will require changes in arch-specific code.

This allows VMAP_STACK support on x86, and can simplify support of architectures that do not have a fixed module region.