path: root/arch
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authorDmitry Vyukov <dvyukov@google.com>2016-03-22 14:27:30 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-03-22 15:36:02 -0700
commit5c9a8750a6409c63a0f01d51a9024861022f6593 (patch)
tree61c5cd39711d26f755a30a7f0fd52f91c1f56387 /arch
parentade356b99a4187578609f2a91c4d2ed88e4e70dc (diff)
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. [akpm@linux-foundation.org: make task_struct.kcov_mode have type `enum kcov_mode'] [akpm@linux-foundation.org: unbreak allmodconfig] [akpm@linux-foundation.org: follow x86 Makefile layout standards] Signed-off-by: Dmitry Vyukov <dvyukov@google.com> Reviewed-by: Kees Cook <keescook@chromium.org> Cc: syzkaller <syzkaller@googlegroups.com> Cc: Vegard Nossum <vegard.nossum@oracle.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Tavis Ormandy <taviso@google.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com> Cc: Kostya Serebryany <kcc@google.com> Cc: Eric Dumazet <edumazet@google.com> Cc: Alexander Potapenko <glider@google.com> Cc: Kees Cook <keescook@google.com> Cc: Bjorn Helgaas <bhelgaas@google.com> Cc: Sasha Levin <sasha.levin@oracle.com> Cc: David Drysdale <drysdale@google.com> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> Cc: Kirill A. Shutemov <kirill@shutemov.name> Cc: Jiri Slaby <jslaby@suse.cz> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch')
10 files changed, 37 insertions, 0 deletions
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 8b680a5cb25b..54478b7635de 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -28,6 +28,7 @@ config X86
+ select ARCH_HAS_KCOV if X86_64
select ARCH_HAS_PMEM_API if X86_64
diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 0bf6749522d9..b1ef9e489084 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -12,6 +12,13 @@
+# 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.
# If you want to preset the SVGA mode, uncomment the next line and
# set SVGA_MODE to whatever number you want.
# Set it to -DSVGA_MODE=NORMAL_VGA if you just want the EGA/VGA mode.
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 5e1d26e09407..6915ff2bd996 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -19,6 +19,9 @@
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index f9fb859c98b9..6874da5f67fc 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -7,6 +7,9 @@ KASAN_SANITIZE := n
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
VDSO64-$(CONFIG_X86_64) := y
VDSOX32-$(CONFIG_X86_X32_ABI) := y
VDSO32-$(CONFIG_X86_32) := y
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index d5fb0871aba3..adaae2c781c1 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -25,6 +25,12 @@ OBJECT_FILES_NON_STANDARD_relocate_kernel_$(BITS).o := y
+# 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.
CFLAGS_irq.o := -I$(src)/../include/asm/trace
obj-y := process_$(BITS).o signal.o
diff --git a/arch/x86/kernel/apic/Makefile b/arch/x86/kernel/apic/Makefile
index 8bb12ddc5db8..8e63ebdcbd0b 100644
--- a/arch/x86/kernel/apic/Makefile
+++ b/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.
obj-$(CONFIG_X86_LOCAL_APIC) += apic.o apic_noop.o ipi.o vector.o
obj-y += hw_nmi.o
diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index 0d373d7affc8..4a8697f7d4ef 100644
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -8,6 +8,10 @@ CFLAGS_REMOVE_common.o = -pg
CFLAGS_REMOVE_perf_event.o = -pg
+# 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)
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index a501fa25da41..72a576752a7e 100644
--- a/arch/x86/lib/Makefile
+++ b/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 $@
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index 67cf2e1e557b..f98913258c63 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -1,3 +1,6 @@
+# Kernel does not boot with instrumentation of tlb.c.
obj-y := init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
pat.o pgtable.o physaddr.o gup.o setup_nx.o
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index 053abe7b0ef7..b95964610ea7 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -9,6 +9,9 @@
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
always := realmode.bin realmode.relocs
wakeup-objs := wakeup_asm.o wakemain.o video-mode.o

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