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authorAlexander Bulekov <alxndr@bu.edu>2020-02-19 23:11:11 -0500
committerStefan Hajnoczi <stefanha@redhat.com>2020-02-22 08:26:48 +0000
commitcb06fdad05f3e546a4e20f1f3c0127f9ae53de1a (patch)
treeb5176e5d8bb8ef7004935b8ca6cbf4c77cf5cf0e
parentd6919e4cb65230b0c8081eb072893d4e8a191a59 (diff)
downloadqemu-arm-cb06fdad05f3e546a4e20f1f3c0127f9ae53de1a.tar.gz
fuzz: support for fork-based fuzzing.
fork() is a simple way to ensure that state does not leak in between fuzzing runs. Unfortunately, the fuzzer mutation engine relies on bitmaps which contain coverage information for each fuzzing run, and these bitmaps should be copied from the child to the parent(where the mutation occurs). These bitmaps are created through compile-time instrumentation and they are not shared with fork()-ed processes, by default. To address this, we create a shared memory region, adjust its size and map it _over_ the counter region. Furthermore, libfuzzer doesn't generally expose the globals that specify the location of the counters/coverage bitmap. As a workaround, we rely on a custom linker script which forces all of the bitmaps we care about to be placed in a contiguous region, which is easy to locate and mmap over. Signed-off-by: Alexander Bulekov <alxndr@bu.edu> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Darren Kenny <darren.kenny@oracle.com> Message-id: 20200220041118.23264-16-alxndr@bu.edu Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
-rw-r--r--tests/qtest/fuzz/Makefile.include5
-rw-r--r--tests/qtest/fuzz/fork_fuzz.c55
-rw-r--r--tests/qtest/fuzz/fork_fuzz.h23
-rw-r--r--tests/qtest/fuzz/fork_fuzz.ld37
4 files changed, 120 insertions, 0 deletions
diff --git a/tests/qtest/fuzz/Makefile.include b/tests/qtest/fuzz/Makefile.include
index 8632bb89f4..a90915d56d 100644
--- a/tests/qtest/fuzz/Makefile.include
+++ b/tests/qtest/fuzz/Makefile.include
@@ -2,5 +2,10 @@ QEMU_PROG_FUZZ=qemu-fuzz-$(TARGET_NAME)$(EXESUF)
fuzz-obj-y += tests/qtest/libqtest.o
fuzz-obj-y += tests/qtest/fuzz/fuzz.o # Fuzzer skeleton
+fuzz-obj-y += tests/qtest/fuzz/fork_fuzz.o
FUZZ_CFLAGS += -I$(SRC_PATH)/tests -I$(SRC_PATH)/tests/qtest
+
+# Linker Script to force coverage-counters into known regions which we can mark
+# shared
+FUZZ_LDFLAGS += -Xlinker -T$(SRC_PATH)/tests/qtest/fuzz/fork_fuzz.ld
diff --git a/tests/qtest/fuzz/fork_fuzz.c b/tests/qtest/fuzz/fork_fuzz.c
new file mode 100644
index 0000000000..2bd0851903
--- /dev/null
+++ b/tests/qtest/fuzz/fork_fuzz.c
@@ -0,0 +1,55 @@
+/*
+ * Fork-based fuzzing helpers
+ *
+ * Copyright Red Hat Inc., 2019
+ *
+ * Authors:
+ * Alexander Bulekov <alxndr@bu.edu>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "fork_fuzz.h"
+
+
+void counter_shm_init(void)
+{
+ char *shm_path = g_strdup_printf("/qemu-fuzz-cntrs.%d", getpid());
+ int fd = shm_open(shm_path, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
+ g_free(shm_path);
+
+ if (fd == -1) {
+ perror("Error: ");
+ exit(1);
+ }
+ if (ftruncate(fd, &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START) == -1) {
+ perror("Error: ");
+ exit(1);
+ }
+ /* Copy what's in the counter region to the shm.. */
+ void *rptr = mmap(NULL ,
+ &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START,
+ PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ memcpy(rptr,
+ &__FUZZ_COUNTERS_START,
+ &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+
+ munmap(rptr, &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+
+ /* And map the shm over the counter region */
+ rptr = mmap(&__FUZZ_COUNTERS_START,
+ &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START,
+ PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0);
+
+ close(fd);
+
+ if (!rptr) {
+ perror("Error: ");
+ exit(1);
+ }
+}
+
+
diff --git a/tests/qtest/fuzz/fork_fuzz.h b/tests/qtest/fuzz/fork_fuzz.h
new file mode 100644
index 0000000000..9ecb8b58ef
--- /dev/null
+++ b/tests/qtest/fuzz/fork_fuzz.h
@@ -0,0 +1,23 @@
+/*
+ * Fork-based fuzzing helpers
+ *
+ * Copyright Red Hat Inc., 2019
+ *
+ * Authors:
+ * Alexander Bulekov <alxndr@bu.edu>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef FORK_FUZZ_H
+#define FORK_FUZZ_H
+
+extern uint8_t __FUZZ_COUNTERS_START;
+extern uint8_t __FUZZ_COUNTERS_END;
+
+void counter_shm_init(void);
+
+#endif
+
diff --git a/tests/qtest/fuzz/fork_fuzz.ld b/tests/qtest/fuzz/fork_fuzz.ld
new file mode 100644
index 0000000000..b23a59f194
--- /dev/null
+++ b/tests/qtest/fuzz/fork_fuzz.ld
@@ -0,0 +1,37 @@
+/* We adjust linker script modification to place all of the stuff that needs to
+ * persist across fuzzing runs into a contiguous seciton of memory. Then, it is
+ * easy to re-map the counter-related memory as shared.
+*/
+
+SECTIONS
+{
+ .data.fuzz_start : ALIGN(4K)
+ {
+ __FUZZ_COUNTERS_START = .;
+ __start___sancov_cntrs = .;
+ *(_*sancov_cntrs);
+ __stop___sancov_cntrs = .;
+
+ /* Lowest stack counter */
+ *(__sancov_lowest_stack);
+ }
+ .data.fuzz_ordered :
+ {
+ /* Coverage counters. They're not necessary for fuzzing, but are useful
+ * for analyzing the fuzzing performance
+ */
+ __start___llvm_prf_cnts = .;
+ *(*llvm_prf_cnts);
+ __stop___llvm_prf_cnts = .;
+
+ /* Internal Libfuzzer TracePC object which contains the ValueProfileMap */
+ FuzzerTracePC*(.bss*);
+ }
+ .data.fuzz_end : ALIGN(4K)
+ {
+ __FUZZ_COUNTERS_END = .;
+ }
+}
+/* Dont overwrite the SECTIONS in the default linker script. Instead insert the
+ * above into the default script */
+INSERT AFTER .data;