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; }