Messages in this thread | | | From | Dmitry Vyukov <> | Date | Tue, 19 Jan 2016 15:20:25 +0100 | Subject | Re: [PATCH v5] kernel: add kcov code coverage |
| |
On Tue, Jan 19, 2016 at 3:17 PM, Kirill A. Shutemov <kirill@shutemov.name> wrote: > On Tue, Jan 19, 2016 at 03:06:10PM +0100, Dmitry Vyukov wrote: >> On Tue, Jan 19, 2016 at 1:42 PM, Dmitry Vyukov <dvyukov@google.com> wrote: >> > kcov provides code coverage collection for coverage-guided fuzzing >> > (randomized testing). Coverage-guided fuzzing is a testing technique >> > that uses coverage feedback to determine new interesting inputs to a >> > system. A notable user-space example is AFL >> > (http://lcamtuf.coredump.cx/afl/). However, this technique is not >> > widely used for kernel testing due to missing compiler and kernel >> > support. >> > >> > kcov does not aim to collect as much coverage as possible. It aims >> > to collect more or less stable coverage that is function of syscall >> > inputs. To achieve this goal it does not collect coverage in >> > soft/hard interrupts and instrumentation of some inherently >> > non-deterministic or non-interesting parts of kernel is disbled >> > (e.g. scheduler, locking). >> > >> > Currently there is a single coverage collection mode (tracing), >> > but the API anticipates additional collection modes. >> > Initially I also implemented a second mode which exposes >> > coverage in a fixed-size hash table of counters (what Quentin >> > used in his original patch). I've dropped the second mode for >> > simplicity. >> > >> > This patch adds the necessary support on kernel side. >> > The complimentary compiler support was added in gcc revision 231296. >> > >> > We've used this support to build syzkaller system call fuzzer, >> > which has found 90 kernel bugs in just 2 months: >> > https://github.com/google/syzkaller/wiki/Found-Bugs >> > We've also found 30+ bugs in our internal systems with syzkaller. >> > Another (yet unexplored) direction where kcov coverage would greatly >> > help is more traditional "blob mutation". For example, mounting >> > a random blob as a filesystem, or receiving a random blob over wire. >> > >> > Why not gcov. Typical fuzzing loop looks as follows: (1) reset >> > coverage, (2) execute a bit of code, (3) collect coverage, repeat. >> > A typical coverage can be just a dozen of basic blocks (e.g. an >> > invalid input). In such context gcov becomes prohibitively expensive >> > as reset/collect coverage steps depend on total number of basic >> > blocks/edges in program (in case of kernel it is about 2M). Cost of >> > kcov depends only on number of executed basic blocks/edges. On top of >> > that, kernel requires per-thread coverage because there are >> > always background threads and unrelated processes that also produce >> > coverage. With inlined gcov instrumentation per-thread coverage is not >> > possible. >> > >> > kcov exposes kernel PCs and control flow to user-space which >> > is insecure. But debugfs should not be mapped as user accessible. >> > >> > Based on a patch by Quentin Casasnovas. >> > Signed-off-by: Dmitry Vyukov <dvyukov@google.com> >> > Reviewed-by: Kees Cook <keescook@chromium.org> >> > --- >> > Anticipating reasonable questions regarding usage of this feature. >> > Quentin Casasnovas and Vegard Nossum also plan to use kcov for >> > coverage-guided fuzzing. Currently they use a custom kernel patch >> > for their fuzzer and found several dozens of bugs. >> > There is also interest from Intel 0-DAY kernel test infrastructure. >> > >> > Based on commit a200dcb34693084e56496960d855afdeaaf9578f. >> > >> > v2: - added note to commit desciption that kcov is insecure, >> > but debugfs should not be mapped as user accessible. >> > - make CONFIG_KCOV depend on CONFIG_ARCH_HAS_KCOV >> > instead of conditional inclusion with if/endif >> > (as per Kees comments). >> > >> > v3: - disabled instrumentation of lib/hweight.c >> > - changed task_struct.kcov_size type to unsigned >> > - moved kcov.c from kernel/kcov/ to kernel/ >> > - fixed multi-line comment formatting >> > - changed BUG_ONs to WARN_ONs >> > - added kcov_get() helper >> > >> > v4: - pre-populate mapping with pages in kcov_mmap() >> > - don't get kcov references on vma open/copy, >> > vma holds a reference to the file which is enough >> > - extend KCOV_INIT_TRACE to support both compressed >> > 4-byte PCs and full 8-byte PCs (it now accepts a struct) >> > - update example in Documentation/kcov.txt >> > >> > v5: - export only unsigned long PCs (no compression to 4 bytes) >> > - remove KCOV dependency on !RANDOMIZE_BASE >> >> >> I've made some measurements. Currently I have ~30MB of coverage data. >> Let's say it will grow 2x over time, that's 60MB. I also use a GC >> language so it actually consumes 2x = 120MB. If PCs are doubled, >> that's 240MB. I think I can live with this. Or I can somehow compress >> PCs to 4 bytes in user-space. >> So changed kcov to expose only unsigned-long-sized PCs as is. This >> makes the interface much cleaner. And also removes all potential >> issues wrt other archs and KASLR (user-space can canonicalize PCs >> using /proc/modules and kaslr base for text). > > I wanted to mention one problem with 'long's: it will not work with 32-bit > userspace on 64-bit kernel. I think we need to have a way to communicate > size of PC.
User program can call uname?
| |