Documentation/dev-tools/kasan.rst - third_party/linux - Git at Google

 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 and s390
 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.
	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 and s390
	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.