On Fri, Sep 21, 2018 at 5:13 PM, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote: > This patch updates KASAN documentation to reflect the addition of the new > tag-based mode. > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > --- > Documentation/dev-tools/kasan.rst | 232 ++++++++++++++++++------------ > 1 file changed, 138 insertions(+), 94 deletions(-) > > diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst > index aabc8738b3d8..a407e18afd32 100644 > --- a/Documentation/dev-tools/kasan.rst > +++ b/Documentation/dev-tools/kasan.rst > @@ -4,15 +4,25 @@ The Kernel Address Sanitizer (KASAN) > Overview > -------- > > -KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides > -a fast and comprehensive solution for finding use-after-free and out-of-bounds > -bugs. > +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 for checking every memory access, > -therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is > -required for detection of out-of-bounds accesses to stack or global variables. > +KASAN uses compile-time instrumentation to insert validity checks before every > +memory access, and therefore requires a compiler version that supports that. > > -Currently KASAN is supported only for the x86_64 and arm64 architectures. > +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 > +3.7.0 or later and it doesn't support detection of out-of-bounds accesses for Note: this should actually be 7.0.0 and not 3.7.0 (as we need rL329612). > +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 and xtensa > +architectures, and tag-based KASAN is supported only for arm64. > > Usage > ----- > @@ -21,12 +31,14 @@ To enable KASAN configure kernel with:: > > CONFIG_KASAN = y > > -and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and > -inline are compiler instrumentation types. The former produces smaller binary > -the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC > -version 5.0 or later. > +and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and > +CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN). > > -KASAN works with both SLUB and SLAB memory allocators. > +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 disable instrumentation for specific files or directories, add a line > @@ -43,85 +55,85 @@ similar to the following to the respective kernel Makefile: > Error reports > ~~~~~~~~~~~~~ > > -A typical out of bounds access report looks like this:: > +A typical out-of-bounds access generic KASAN report looks like this:: > > ================================================================== > - BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3 > - Write of size 1 by task modprobe/1689 > - ============================================================================= > - BUG kmalloc-128 (Not tainted): kasan error > - ----------------------------------------------------------------------------- > - > - Disabling lock debugging due to kernel taint > - INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689 > - __slab_alloc+0x4b4/0x4f0 > - kmem_cache_alloc_trace+0x10b/0x190 > - kmalloc_oob_right+0x3d/0x75 [test_kasan] > - init_module+0x9/0x47 [test_kasan] > - do_one_initcall+0x99/0x200 > - load_module+0x2cb3/0x3b20 > - SyS_finit_module+0x76/0x80 > - system_call_fastpath+0x12/0x17 > - INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080 > - INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720 > - > - Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ > - Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > - Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. > - Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........ > - Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ > - CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98 > - Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014 > - ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78 > - ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8 > - ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558 > + 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: > - [<ffffffff81cc68ae>] dump_stack+0x46/0x58 > - [<ffffffff811fd848>] print_trailer+0xf8/0x160 > - [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] > - [<ffffffff811ff0f5>] object_err+0x35/0x40 > - [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan] > - [<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0 > - [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40 > - [<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40 > - [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40 > - [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] > - [<ffffffff8120a995>] __asan_store1+0x75/0xb0 > - [<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan] > - [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan] > - [<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan] > - [<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan] > - [<ffffffff810002d9>] do_one_initcall+0x99/0x200 > - [<ffffffff811e4e5c>] ? __vunmap+0xec/0x160 > - [<ffffffff81114f63>] load_module+0x2cb3/0x3b20 > - [<ffffffff8110fd70>] ? m_show+0x240/0x240 > - [<ffffffff81115f06>] SyS_finit_module+0x76/0x80 > - [<ffffffff81cd3129>] system_call_fastpath+0x12/0x17 > + 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: > - ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > - ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc > - ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > - ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > - ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00 > - >ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc > - ^ > - ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > - ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > - ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb > - ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > - ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > + 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 discribe what kind of bug happened and what kind of > -access caused it. It's followed by the description of the accessed slub object > -(see 'SLUB Debug output' section in Documentation/vm/slub.rst for details) and > -the description of the accessed memory page. > +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. > @@ -138,18 +150,24 @@ 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 check shadow memory on each > -memory access. > +to access, and use compile-time instrumentation to insert checks of shadow > +memory on each memory access. > > -AddressSanitizer dedicates 1/8 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. > +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:: > @@ -162,12 +180,38 @@ address:: > > where ``KASAN_SHADOW_SCALE_SHIFT = 3``. > > -Compile-time instrumentation used for checking memory accesses. 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. > +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. > -- > 2.19.0.444.g18242da7ef-goog >
