Permalink
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
307 lines (191 sloc) 7.51 KB
/*
american fuzzy lop - high-performance binary-only instrumentation
-----------------------------------------------------------------
Written by Andrew Griffiths <agriffiths@google.com> and
Michal Zalewski <lcamtuf@google.com>
Idea & design very much by Andrew Griffiths.
Copyright 2015, 2016, 2017 Google Inc. All rights reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at:
http://www.apache.org/licenses/LICENSE-2.0
This code is a shim patched into the separately-distributed source
code of QEMU 2.10.0. It leverages the built-in QEMU tracing functionality
to implement AFL-style instrumentation and to take care of the remaining
parts of the AFL fork server logic.
The resulting QEMU binary is essentially a standalone instrumentation
tool; for an example of how to leverage it for other purposes, you can
have a look at afl-showmap.c.
*/
#include <sys/shm.h>
#include "../../config.h"
/***************************
* VARIOUS AUXILIARY STUFF *
***************************/
/* A snippet patched into tb_find_slow to inform the parent process that
we have hit a new block that hasn't been translated yet, and to tell
it to translate within its own context, too (this avoids translation
overhead in the next forked-off copy). */
#define AFL_QEMU_CPU_SNIPPET1 do { \
afl_request_tsl(pc, cs_base, flags); \
} while (0)
/* This snippet kicks in when the instruction pointer is positioned at
_start and does the usual forkserver stuff, not very different from
regular instrumentation injected via afl-as.h. */
#define AFL_QEMU_CPU_SNIPPET2 do { \
if(itb->pc == afl_entry_point) { \
afl_setup(); \
afl_forkserver(cpu); \
} \
afl_maybe_log(itb->pc); \
} while (0)
/* We use one additional file descriptor to relay "needs translation"
messages between the child and the fork server. */
#define TSL_FD (FORKSRV_FD - 1)
/* This is equivalent to afl-as.h: */
static unsigned char *afl_area_ptr;
/* Exported variables populated by the code patched into elfload.c: */
abi_ulong afl_entry_point, /* ELF entry point (_start) */
afl_start_code, /* .text start pointer */
afl_end_code; /* .text end pointer */
/* Set in the child process in forkserver mode: */
static unsigned char afl_fork_child;
unsigned int afl_forksrv_pid;
/* Instrumentation ratio: */
static unsigned int afl_inst_rms = MAP_SIZE;
/* Function declarations. */
static void afl_setup(void);
static void afl_forkserver(CPUState*);
static inline void afl_maybe_log(abi_ulong);
static void afl_wait_tsl(CPUState*, int);
static void afl_request_tsl(target_ulong, target_ulong, uint64_t);
/* Data structure passed around by the translate handlers: */
struct afl_tsl {
target_ulong pc;
target_ulong cs_base;
uint64_t flags;
};
/* Some forward decls: */
TranslationBlock *tb_htable_lookup(CPUState*, target_ulong, target_ulong, uint32_t);
static inline TranslationBlock *tb_find(CPUState*, TranslationBlock*, int);
/*************************
* ACTUAL IMPLEMENTATION *
*************************/
/* Set up SHM region and initialize other stuff. */
static void afl_setup(void) {
char *id_str = getenv(SHM_ENV_VAR),
*inst_r = getenv("AFL_INST_RATIO");
int shm_id;
if (inst_r) {
unsigned int r;
r = atoi(inst_r);
if (r > 100) r = 100;
if (!r) r = 1;
afl_inst_rms = MAP_SIZE * r / 100;
}
if (id_str) {
shm_id = atoi(id_str);
afl_area_ptr = shmat(shm_id, NULL, 0);
if (afl_area_ptr == (void*)-1) exit(1);
/* With AFL_INST_RATIO set to a low value, we want to touch the bitmap
so that the parent doesn't give up on us. */
if (inst_r) afl_area_ptr[0] = 1;
}
if (getenv("AFL_INST_LIBS")) {
afl_start_code = 0;
afl_end_code = (abi_ulong)-1;
}
/* pthread_atfork() seems somewhat broken in util/rcu.c, and I'm
not entirely sure what is the cause. This disables that
behaviour, and seems to work alright? */
rcu_disable_atfork();
}
/* Fork server logic, invoked once we hit _start. */
static void afl_forkserver(CPUState *cpu) {
static unsigned char tmp[4];
if (!afl_area_ptr) return;
/* Tell the parent that we're alive. If the parent doesn't want
to talk, assume that we're not running in forkserver mode. */
if (write(FORKSRV_FD + 1, tmp, 4) != 4) return;
afl_forksrv_pid = getpid();
/* All right, let's await orders... */
while (1) {
pid_t child_pid;
int status, t_fd[2];
/* Whoops, parent dead? */
if (read(FORKSRV_FD, tmp, 4) != 4) exit(2);
/* Establish a channel with child to grab translation commands. We'll
read from t_fd[0], child will write to TSL_FD. */
if (pipe(t_fd) || dup2(t_fd[1], TSL_FD) < 0) exit(3);
close(t_fd[1]);
child_pid = fork();
if (child_pid < 0) exit(4);
if (!child_pid) {
/* Child process. Close descriptors and run free. */
afl_fork_child = 1;
close(FORKSRV_FD);
close(FORKSRV_FD + 1);
close(t_fd[0]);
return;
}
/* Parent. */
close(TSL_FD);
if (write(FORKSRV_FD + 1, &child_pid, 4) != 4) exit(5);
/* Collect translation requests until child dies and closes the pipe. */
afl_wait_tsl(cpu, t_fd[0]);
/* Get and relay exit status to parent. */
if (waitpid(child_pid, &status, 0) < 0) exit(6);
if (write(FORKSRV_FD + 1, &status, 4) != 4) exit(7);
}
}
/* The equivalent of the tuple logging routine from afl-as.h. */
static inline void afl_maybe_log(abi_ulong cur_loc) {
static __thread abi_ulong prev_loc;
/* Optimize for cur_loc > afl_end_code, which is the most likely case on
Linux systems. */
if (cur_loc > afl_end_code || cur_loc < afl_start_code || !afl_area_ptr)
return;
/* Looks like QEMU always maps to fixed locations, so ASAN is not a
concern. Phew. But instruction addresses may be aligned. Let's mangle
the value to get something quasi-uniform. */
cur_loc = (cur_loc >> 4) ^ (cur_loc << 8);
cur_loc &= MAP_SIZE - 1;
/* Implement probabilistic instrumentation by looking at scrambled block
address. This keeps the instrumented locations stable across runs. */
if (cur_loc >= afl_inst_rms) return;
afl_area_ptr[cur_loc ^ prev_loc]++;
prev_loc = cur_loc >> 1;
}
/* This code is invoked whenever QEMU decides that it doesn't have a
translation of a particular block and needs to compute it. When this happens,
we tell the parent to mirror the operation, so that the next fork() has a
cached copy. */
static void afl_request_tsl(target_ulong pc, target_ulong cb, uint64_t flags) {
struct afl_tsl t;
if (!afl_fork_child) return;
t.pc = pc;
t.cs_base = cb;
t.flags = flags;
if (write(TSL_FD, &t, sizeof(struct afl_tsl)) != sizeof(struct afl_tsl))
return;
}
/* This is the other side of the same channel. Since timeouts are handled by
afl-fuzz simply killing the child, we can just wait until the pipe breaks. */
static void afl_wait_tsl(CPUState *cpu, int fd) {
struct afl_tsl t;
TranslationBlock *tb;
while (1) {
/* Broken pipe means it's time to return to the fork server routine. */
if (read(fd, &t, sizeof(struct afl_tsl)) != sizeof(struct afl_tsl))
break;
tb = tb_htable_lookup(cpu, t.pc, t.cs_base, t.flags);
if(!tb) {
mmap_lock();
tb_lock();
tb_gen_code(cpu, t.pc, t.cs_base, t.flags, 0);
mmap_unlock();
tb_unlock();
}
}
close(fd);
}