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Subjectx86: warning in unwind_get_return_address
Hi,

I've got the following warning while running the syzkaller fuzzer:

WARNING: unrecognized kernel stack return address ffffffffa0000001 at
ffff88006377fa18 in a.out:4467

By adding a BUG() to unwind_get_return_address() I was able to capture
the stack trace (see below). Looks like unwind_get_return_address()
gets called when KASAN tries to unwind the stack to save the stack
trace.

A reproducer is attached. CONFIG_KASAN=y is most likely needed for it to work.

On commit e93b1cc8a8965da137ffea0b88e5f62fa1d2a9e6 (Dec 19).

------------[ cut here ]------------
kernel BUG at arch/x86/kernel/unwind_frame.c:27!
invalid opcode: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 1 PID: 4467 Comm: a.out Not tainted 4.9.0+ #53
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff8800665b9600 task.stack: ffff880063778000
RIP: 0010:unwind_get_return_address+0xcc/0x1b0 arch/x86/kernel/unwind_frame.c:24
RSP: 0018:ffff88006cb06fa0 EFLAGS: 00010286
RAX: 0000000000000064 RBX: ffff8800665b9600 RCX: 0000000000000000
RDX: 0000000000000100 RSI: ffff88006cb15f08 RDI: ffffed000d960de6
RBP: ffff88006cb06fb8 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000009 R11: 0000000000000000 R12: ffffffffa0000001
R13: ffff88006377fa18 R14: ffff8800665b9600 R15: 0000000000000246
FS: 00007f7c347947c0(0000) GS:ffff88006cb00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020004fc8 CR3: 0000000062668000 CR4: 00000000000006e0
Call Trace:
<IRQ>
__save_stack_trace+0x7e/0xd0 arch/x86/kernel/stacktrace.c:43
save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
save_stack+0x43/0xd0 mm/kasan/kasan.c:502
set_track mm/kasan/kasan.c:514
kasan_slab_free+0x73/0xc0 mm/kasan/kasan.c:578
slab_free_hook mm/slub.c:1352
slab_free_freelist_hook mm/slub.c:1374
slab_free mm/slub.c:2951
kmem_cache_free+0xb2/0x2c0 mm/slub.c:2973
file_free_rcu+0x6d/0xa0 fs/file_table.c:49
__rcu_reclaim kernel/rcu/rcu.h:118
rcu_do_batch.isra.67+0x900/0xc50 kernel/rcu/tree.c:2777
invoke_rcu_callbacks kernel/rcu/tree.c:3040
__rcu_process_callbacks kernel/rcu/tree.c:3007
rcu_process_callbacks+0x2b7/0xba0 kernel/rcu/tree.c:3024
__do_softirq+0x2fb/0xb7d kernel/softirq.c:284
invoke_softirq kernel/softirq.c:364
irq_exit+0x19e/0x1d0 kernel/softirq.c:405
exiting_irq ./arch/x86/include/asm/apic.h:658
smp_apic_timer_interrupt+0x76/0xa0 arch/x86/kernel/apic/apic.c:961
apic_timer_interrupt+0x93/0xa0
RIP: 0010:0xffffffffa0000001
RSP: 0018:ffff88006377fa48 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff10
RAX: ffff8800665b9600 RBX: ffff880066471400 RCX: 1ffffffff0bc9005
RDX: 0000000000000000 RSI: ffff88006377fe78 RDI: ffff880066471400
RBP: ffff88006377fb00 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff1000c6eff4f
R13: ffffffff85e47fc0 R14: 0000000000000000 R15: ffff880066471428
</IRQ>
Code: 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 eb 00 00 00 4d 8b 65
00 4c 89 e7 e8 21 75 0c 00 85 c0 75 0e 80 3d 44 fc c9 04 00 74 4f <0f>
0b 45 31 e4 5b 4c 89 e0 41 5c 41 5d 5d c3 48 8d 7b 38 48 b8
RIP: unwind_get_return_address+0xcc/0x1b0 RSP: ffff88006cb06fa0
---[ end trace a4f6a441af47c2dc ]---
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: disabled
---[ end Kernel panic - not syncing: Fatal exception in interrupt
// autogenerated by syzkaller (http://github.com/google/syzkaller)

#ifndef __NR_mmap
#define __NR_mmap 9
#endif
#ifndef __NR_socket
#define __NR_socket 41
#endif
#ifndef __NR_sendmsg
#define __NR_sendmsg 46
#endif

#define _GNU_SOURCE

#include <sys/ioctl.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/resource.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>

#include <linux/capability.h>
#include <linux/if.h>
#include <linux/if_tun.h>
#include <linux/sched.h>
#include <net/if_arp.h>

#include <assert.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <pthread.h>
#include <setjmp.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

const int kFailStatus = 67;
const int kErrorStatus = 68;
const int kRetryStatus = 69;

__attribute__((noreturn)) void fail(const char* msg, ...)
{
int e = errno;
fflush(stdout);
va_list args;
va_start(args, msg);
vfprintf(stderr, msg, args);
va_end(args);
fprintf(stderr, " (errno %d)\n", e);
exit(kFailStatus);
}

__attribute__((noreturn)) void exitf(const char* msg, ...)
{
int e = errno;
fflush(stdout);
va_list args;
va_start(args, msg);
vfprintf(stderr, msg, args);
va_end(args);
fprintf(stderr, " (errno %d)\n", e);
exit(kRetryStatus);
}

static int flag_debug;

void debug(const char* msg, ...)
{
if (!flag_debug)
return;
va_list args;
va_start(args, msg);
vfprintf(stdout, msg, args);
va_end(args);
fflush(stdout);
}

__thread int skip_segv;
__thread jmp_buf segv_env;

static void segv_handler(int sig, siginfo_t* info, void* uctx)
{
if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED))
_longjmp(segv_env, 1);
exit(sig);
}

static void install_segv_handler()
{
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_sigaction = segv_handler;
sa.sa_flags = SA_NODEFER | SA_SIGINFO;
sigaction(SIGSEGV, &sa, NULL);
sigaction(SIGBUS, &sa, NULL);
}

#define NONFAILING(...) \
{ \
__atomic_fetch_add(&skip_segv, 1, __ATOMIC_SEQ_CST); \
if (_setjmp(segv_env) == 0) { \
__VA_ARGS__; \
} \
__atomic_fetch_sub(&skip_segv, 1, __ATOMIC_SEQ_CST); \
}

static uintptr_t execute_syscall(int nr, uintptr_t a0, uintptr_t a1,
uintptr_t a2, uintptr_t a3,
uintptr_t a4, uintptr_t a5,
uintptr_t a6, uintptr_t a7,
uintptr_t a8)
{
switch (nr) {
default:
return syscall(nr, a0, a1, a2, a3, a4, a5);
}
}

static void setup_main_process(uint64_t pid, bool enable_tun)
{
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = SIG_IGN;
syscall(SYS_rt_sigaction, 0x20, &sa, NULL, 8);
syscall(SYS_rt_sigaction, 0x21, &sa, NULL, 8);
install_segv_handler();

char tmpdir_template[] = "./syzkaller.XXXXXX";
char* tmpdir = mkdtemp(tmpdir_template);
if (!tmpdir)
fail("failed to mkdtemp");
if (chmod(tmpdir, 0777))
fail("failed to chmod");
if (chdir(tmpdir))
fail("failed to chdir");
}

static void loop();

static void sandbox_common()
{
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
setpgrp();
setsid();

struct rlimit rlim;
rlim.rlim_cur = rlim.rlim_max = 128 << 20;
setrlimit(RLIMIT_AS, &rlim);
rlim.rlim_cur = rlim.rlim_max = 1 << 20;
setrlimit(RLIMIT_FSIZE, &rlim);
rlim.rlim_cur = rlim.rlim_max = 1 << 20;
setrlimit(RLIMIT_STACK, &rlim);
rlim.rlim_cur = rlim.rlim_max = 0;
setrlimit(RLIMIT_CORE, &rlim);

unshare(CLONE_NEWNS);
unshare(CLONE_NEWIPC);
unshare(CLONE_IO);
}

static int do_sandbox_none()
{
int pid = fork();
if (pid)
return pid;
sandbox_common();
loop();
exit(1);
}

static void remove_dir(const char* dir)
{
DIR* dp;
struct dirent* ep;
int iter = 0;
retry:
dp = opendir(dir);
if (dp == NULL) {
if (errno == EMFILE) {
exitf("opendir(%s) failed due to NOFILE, exiting");
}
exitf("opendir(%s) failed", dir);
}
while ((ep = readdir(dp))) {
if (strcmp(ep->d_name, ".") == 0 || strcmp(ep->d_name, "..") == 0)
continue;
char filename[FILENAME_MAX];
snprintf(filename, sizeof(filename), "%s/%s", dir, ep->d_name);
struct stat st;
if (lstat(filename, &st))
exitf("lstat(%s) failed", filename);
if (S_ISDIR(st.st_mode)) {
remove_dir(filename);
continue;
}
int i;
for (i = 0;; i++) {
debug("unlink(%s)\n", filename);
if (unlink(filename) == 0)
break;
if (errno == EROFS) {
debug("ignoring EROFS\n");
break;
}
if (errno != EBUSY || i > 100)
exitf("unlink(%s) failed", filename);
debug("umount(%s)\n", filename);
if (umount2(filename, MNT_DETACH))
exitf("umount(%s) failed", filename);
}
}
closedir(dp);
int i;
for (i = 0;; i++) {
debug("rmdir(%s)\n", dir);
if (rmdir(dir) == 0)
break;
if (i < 100) {
if (errno == EROFS) {
debug("ignoring EROFS\n");
break;
}
if (errno == EBUSY) {
debug("umount(%s)\n", dir);
if (umount2(dir, MNT_DETACH))
exitf("umount(%s) failed", dir);
continue;
}
if (errno == ENOTEMPTY) {
if (iter < 100) {
iter++;
goto retry;
}
}
}
exitf("rmdir(%s) failed", dir);
}
}

static uint64_t current_time_ms()
{
struct timespec ts;

if (clock_gettime(CLOCK_MONOTONIC, &ts))
fail("clock_gettime failed");
return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void test();

void loop()
{
int iter;
for (iter = 0;; iter++) {
char cwdbuf[256];
sprintf(cwdbuf, "./%d", iter);
if (mkdir(cwdbuf, 0777))
fail("failed to mkdir");
int pid = fork();
if (pid < 0)
fail("clone failed");
if (pid == 0) {
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
setpgrp();
if (chdir(cwdbuf))
fail("failed to chdir");
test();
exit(0);
}
int status = 0;
uint64_t start = current_time_ms();
for (;;) {
int res = waitpid(pid, &status, __WALL | WNOHANG);
int errno0 = errno;
if (res == pid)
break;
usleep(1000);
if (current_time_ms() - start > 5 * 1000) {
kill(-pid, SIGKILL);
kill(pid, SIGKILL);
waitpid(pid, &status, __WALL);
break;
}
}
remove_dir(cwdbuf);
}
}

long r[11];
void test()
{
memset(r, -1, sizeof(r));
r[0] = execute_syscall(__NR_mmap, 0x20000000ul, 0x6000ul, 0x3ul,
0x32ul, 0xfffffffffffffffful, 0x0ul, 0, 0, 0);
r[1] = execute_syscall(__NR_socket, 0xaul, 0x806ul, 0x0ul, 0, 0, 0, 0,
0, 0);
NONFAILING(*(uint64_t*)0x20004fc8 = (uint64_t)0x20004000);
NONFAILING(*(uint32_t*)0x20004fd0 = (uint32_t)0x2);
NONFAILING(*(uint64_t*)0x20004fd8 = (uint64_t)0x20004000);
NONFAILING(*(uint64_t*)0x20004fe0 = (uint64_t)0x0);
NONFAILING(*(uint64_t*)0x20004fe8 = (uint64_t)0x20005000);
NONFAILING(*(uint64_t*)0x20004ff0 = (uint64_t)0x0);
NONFAILING(*(uint32_t*)0x20004ff8 = (uint32_t)0x0);
NONFAILING(*(uint16_t*)0x20004000 = (uint16_t)0x0);
r[10] = execute_syscall(__NR_sendmsg, r[1], 0x20004fc8ul, 0x0ul, 0, 0,
0, 0, 0, 0);
}
int main()
{
setup_main_process(0, false);
int pid = do_sandbox_none();
int status = 0;
while (waitpid(pid, &status, __WALL) != pid) {
}
return 0;
}
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