kernel: add kcov code coverage

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 a200dcb.

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
    - update example in Documentation/kcov.txt
    - rename kcov_mode_trace to KCOV_MODE_TRACE
    - warn about failed vm_insert_page()
    - ensure that arg for KCOV_ENABLE/KCOV_DISABLE is 0

Documentation/kcov.txt

@@ -0,0 +1,112 @@
+kcov: code coverage for fuzzing
+===============================
+
+kcov exposes kernel code coverage information in a form suitable for coverage-
+guided fuzzing (randomized testing). Coverage data of a running kernel is
+exported via the "kcov" debugfs file. Coverage collection is enabled on a task
+basis, and thus it can capture precise coverage of a single system call.
+
+Note that 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 parts of kernel is
+disbled (e.g. scheduler, locking).
+
+Usage:
+======
+
+Configure kernel with:
+
+        CONFIG_KCOV=y
+        CONFIG_DEBUG_FS=y
+
+CONFIG_KCOV requires gcc built on revision 231296 or later.
+Profiling data will only become accessible once debugfs has been mounted:
+
+        mount -t debugfs none /sys/kernel/debug
+
+The following program demonstrates kcov usage from within a test program:
+
+#include <stdio.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
+#define KCOV_ENABLE			_IO('c', 100)
+#define KCOV_DISABLE			_IO('c', 101)
+#define COVER_SIZE			(64<<10)
+
+int main(int argc, char **argv)
+{
+	int fd;
+	unsigned long *cover, n, i;
+
+	/* A single fd descriptor allows coverage collection on a single
+	 * thread.
+	 */
+	fd = open("/sys/kernel/debug/kcov", O_RDWR);
+	if (fd == -1)
+		perror("open"), exit(1);
+	/* Setup trace mode and trace size. */
+	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+		perror("ioctl"), exit(1);
+	/* Mmap buffer shared between kernel- and user-space. */
+	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if ((void*)cover == MAP_FAILED)
+		perror("mmap"), exit(1);
+	/* Enable coverage collection on the current thread. */
+	if (ioctl(fd, KCOV_ENABLE, 0))
+		perror("ioctl"), exit(1);
+	/* Reset coverage from the tail of the ioctl() call. */
+	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
+	/* That's the target syscal call. */
+	read(-1, NULL, 0);
+	/* Read number of PCs collected. */
+	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+	for (i = 0; i < n; i++)
+		printf("0x%lx\n", cover[i + 1]);
+	/* Disable coverage collection for the current thread. After this call
+	 * coverage can be enabled for a different thread.
+	 */
+	if (ioctl(fd, KCOV_DISABLE, 0))
+		perror("ioctl"), exit(1);
+	/* Free resources. */
+	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+		perror("munmap"), exit(1);
+	if (close(fd))
+		perror("close"), exit(1);
+	return 0;
+}
+
+After piping through addr2line output of the program looks as follows:
+
+SyS_read
+fs/read_write.c:562
+__fdget_pos
+fs/file.c:774
+__fget_light
+fs/file.c:746
+__fget_light
+fs/file.c:750
+__fget_light
+fs/file.c:760
+__fdget_pos
+fs/file.c:784
+SyS_read
+fs/read_write.c:562
+
+If a program needs to collect coverage from several threads (independently),
+it needs to open /sys/kernel/debug/kcov in each thread separately.
+
+The interface is fine-grained to allow efficient forking of test processes.
+That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
+mmaps coverage buffer and then forks child processes in a loop. Child processes
+only need to enable coverage (disable happens automatically on thread end).

Makefile

@@ -365,6 +365,7 @@ LDFLAGS_MODULE  =
 CFLAGS_KERNEL	=
 AFLAGS_KERNEL	=
 CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage
+CFLAGS_KCOV	= -fsanitize-coverage=trace-pc
 
 
 # Use USERINCLUDE when you must reference the UAPI directories only.
@@ -411,7 +412,7 @@ export MAKE AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
 export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS LDFLAGS
-export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KASAN CFLAGS_UBSAN
+export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KCOV CFLAGS_KASAN CFLAGS_UBSAN
 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
@@ -673,6 +674,13 @@ endif
 endif
 KBUILD_CFLAGS += $(stackp-flag)
 
+ifdef CONFIG_KCOV
+  ifeq ($(call cc-option, $(CFLAGS_KCOV)),)
+    $(warning Cannot use CONFIG_KCOV: \
+             -fsanitize-coverage=trace-pc is not supported by compiler)
+  endif
+endif
+
 ifeq ($(cc-name),clang)
 KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)

arch/x86/Kconfig

@@ -28,6 +28,7 @@ config X86
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_GCOV_PROFILE_ALL
+	select ARCH_HAS_KCOV			if X86_64
 	select ARCH_HAS_PMEM_API		if X86_64
 	select ARCH_HAS_MMIO_FLUSH
 	select ARCH_HAS_SG_CHAIN

arch/x86/boot/Makefile

@@ -10,6 +10,12 @@
 #
 
 KASAN_SANITIZE := n
+# Kernel does not boot with kcov instrumentation here.
+# One of the problems observed was insertion of __sanitizer_cov_trace_pc()
+# callback into middle of per-cpu data enabling code. Thus the callback observed
+# inconsistent state and crashed. We are interested mostly in syscall coverage,
+# so boot code is not interesting anyway.
+KCOV_INSTRUMENT := n
 
 # If you want to preset the SVGA mode, uncomment the next line and
 # set SVGA_MODE to whatever number you want.

arch/x86/boot/compressed/Makefile

@@ -17,6 +17,8 @@
 #	compressed vmlinux.bin.all + u32 size of vmlinux.bin.all
 
 KASAN_SANITIZE := n
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT := n
 
 targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
 	vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4

arch/x86/entry/vdso/Makefile

@@ -5,6 +5,8 @@
 KBUILD_CFLAGS += $(DISABLE_LTO)
 KASAN_SANITIZE := n
 UBSAN_SANITIZE := n
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT := n
 
 VDSO64-$(CONFIG_X86_64)		:= y
 VDSOX32-$(CONFIG_X86_X32_ABI)	:= y

arch/x86/kernel/Makefile

@@ -19,6 +19,11 @@ endif
 KASAN_SANITIZE_head$(BITS).o := n
 KASAN_SANITIZE_dumpstack.o := n
 KASAN_SANITIZE_dumpstack_$(BITS).o := n
+# If instrumentation of this dir is enabled, boot hangs during first second.
+# Probably could be more selective here, but note that files related to irqs,
+# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
+# non-deterministic coverage.
+KCOV_INSTRUMENT := n
 
 CFLAGS_irq.o := -I$(src)/../include/asm/trace
 

arch/x86/kernel/apic/Makefile

@@ -2,6 +2,10 @@
 # Makefile for local APIC drivers and for the IO-APIC code
 #
 
+# Leads to non-deterministic coverage that is not a function of syscall inputs.
+# In particualr, smp_apic_timer_interrupt() is called in random places.
+KCOV_INSTRUMENT := n
+
 obj-$(CONFIG_X86_LOCAL_APIC)	+= apic.o apic_noop.o ipi.o vector.o
 obj-y				+= hw_nmi.o
 

arch/x86/kernel/cpu/Makefile

@@ -8,6 +8,10 @@ CFLAGS_REMOVE_common.o = -pg
 CFLAGS_REMOVE_perf_event.o = -pg
 endif
 
+# If these files are instrumented, boot hangs during the first second.
+KCOV_INSTRUMENT_common.o := n
+KCOV_INSTRUMENT_perf_event.o := n
+
 # Make sure load_percpu_segment has no stackprotector
 nostackp := $(call cc-option, -fno-stack-protector)
 CFLAGS_common.o		:= $(nostackp)

arch/x86/lib/Makefile

@@ -2,6 +2,9 @@
 # Makefile for x86 specific library files.
 #
 
+# Produces uninteresting flaky coverage.
+KCOV_INSTRUMENT_delay.o := n
+
 inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
 inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt
 quiet_cmd_inat_tables = GEN     $@

arch/x86/mm/Makefile

@@ -1,3 +1,6 @@
+# Kernel does not boot with instrumentation of tlb.c.
+KCOV_INSTRUMENT_tlb.o := n
+
 obj-y	:=  init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
 	    pat.o pgtable.o physaddr.o gup.o setup_nx.o
 

arch/x86/realmode/rm/Makefile

@@ -7,6 +7,8 @@
 #
 #
 KASAN_SANITIZE := n
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT := n
 
 always := realmode.bin realmode.relocs
 

include/linux/kcov.h

@@ -0,0 +1,19 @@
+#ifndef _LINUX_KCOV_H
+#define _LINUX_KCOV_H
+
+#include <uapi/linux/kcov.h>
+
+struct task_struct;
+
+#ifdef CONFIG_KCOV
+
+void kcov_task_init(struct task_struct *t);
+void kcov_task_exit(struct task_struct *t);
+
+#else
+
+static inline void kcov_task_init(struct task_struct *t) {}
+static inline void kcov_task_exit(struct task_struct *t) {}
+
+#endif /* CONFIG_KCOV */
+#endif /* _LINUX_KCOV_H */

include/linux/sched.h

@@ -1811,6 +1811,16 @@ struct task_struct {
 	/* bitmask and counter of trace recursion */
 	unsigned long trace_recursion;
 #endif /* CONFIG_TRACING */
+#ifdef CONFIG_KCOV
+	/* Coverage collection mode enabled for this task (0 if disabled). */
+	int		kcov_mode;
+	/* Size of the kcov_area. */
+	unsigned	kcov_size;
+	/* Buffer for coverage collection. */
+	void		*kcov_area;
+	/* kcov desciptor wired with this task or NULL. */
+	struct kcov	*kcov;
+#endif
 #ifdef CONFIG_MEMCG
 	struct mem_cgroup *memcg_in_oom;
 	gfp_t memcg_oom_gfp_mask;

include/uapi/linux/kcov.h

@@ -0,0 +1,10 @@
+#ifndef _LINUX_KCOV_IOCTLS_H
+#define _LINUX_KCOV_IOCTLS_H
+
+#include <linux/types.h>
+
+#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
+#define KCOV_ENABLE			_IO('c', 100)
+#define KCOV_DISABLE			_IO('c', 101)
+
+#endif /* _LINUX_KCOV_IOCTLS_H */

kernel/Makefile

@@ -19,6 +19,17 @@ CFLAGS_REMOVE_cgroup-debug.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_irq_work.o = $(CC_FLAGS_FTRACE)
 endif
 
+# Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()
+# in coverage traces.
+KCOV_INSTRUMENT_softirq.o := n
+# These are called from save_stack_trace() on slub debug path,
+# and produce insane amounts of uninteresting coverage.
+KCOV_INSTRUMENT_module.o := n
+KCOV_INSTRUMENT_extable.o := n
+# Don't self-instrument.
+KCOV_INSTRUMENT_kcov.o := n
+KASAN_SANITIZE_kcov.o := n
+
 # cond_syscall is currently not LTO compatible
 CFLAGS_sys_ni.o = $(DISABLE_LTO)
 
@@ -69,6 +80,7 @@ obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
 obj-$(CONFIG_AUDIT_WATCH) += audit_watch.o audit_fsnotify.o
 obj-$(CONFIG_AUDIT_TREE) += audit_tree.o
 obj-$(CONFIG_GCOV_KERNEL) += gcov/
+obj-$(CONFIG_KCOV) += kcov.o
 obj-$(CONFIG_KPROBES) += kprobes.o
 obj-$(CONFIG_KGDB) += debug/
 obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o

kernel/exit.c

@@ -53,6 +53,7 @@
 #include <linux/oom.h>
 #include <linux/writeback.h>
 #include <linux/shm.h>
+#include <linux/kcov.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -655,6 +656,7 @@ void do_exit(long code)
 	TASKS_RCU(int tasks_rcu_i);
 
 	profile_task_exit(tsk);
+	kcov_task_exit(tsk);
 
 	WARN_ON(blk_needs_flush_plug(tsk));
 

kernel/fork.c

@@ -75,6 +75,7 @@
 #include <linux/aio.h>
 #include <linux/compiler.h>
 #include <linux/sysctl.h>
+#include <linux/kcov.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -384,6 +385,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 
 	account_kernel_stack(ti, 1);
 
+	kcov_task_init(tsk);
+
 	return tsk;
 
 free_ti: