Bug in __mmdrop() triggered by io-uring on v5.11-rc1

From: Christian Brauner
Date: Mon Dec 28 2020 - 11:55:38 EST

Hey everyone,

The following oops can be triggered on a pristine v5.11-rc1 which I discovered
while rebasing my idmapped mount patchset onto v5.11-rc1:

[ 577.716339][ T7216] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS 0.0.0 02/06/2015
[ 577.718584][ T7216] Call Trace:
[ 577.719357][ T7216] dump_stack+0x10b/0x167
[ 577.720505][ T7216] panic+0x347/0x783
[ 577.721588][ T7216] ? print_oops_end_marker.cold+0x15/0x15
[ 577.723502][ T7216] ? __warn.cold+0x5/0x2f
[ 577.725079][ T7216] ? __mmdrop+0x30c/0x400
[ 577.736066][ T7216] __warn.cold+0x20/0x2f
[ 577.745503][ T7216] ? __mmdrop+0x30c/0x400
[ 577.755101][ T7216] report_bug+0x277/0x300

f2-vm login: [ 577.764873][ T7216] handle_bug+0x3c/0x60
[ 577.773982][ T7216] exc_invalid_op+0x18/0x50
[ 577.786341][ T7216] asm_exc_invalid_op+0x12/0x20
[ 577.795500][ T7216] RIP: 0010:__mmdrop+0x30c/0x400
[ 577.804426][ T7216] Code: 00 00 4c 89 ef e8 64 61 8c 02 eb 82 e8 dd 48 32 00 4c 89 e7 e8 35 97 2e 00 e9 70 ff ff ff e8 cb 48 32 00 0f 0b e8 c4 48 32 00 <0f> 0b e9 51 fd ff ff e8 b8 48 32 00 0f 0b e9 82 fd ff ff e8 ac 48
[ 577.826526][ T7216] RSP: 0018:ffffc900073676d8 EFLAGS: 00010246
[ 577.836448][ T7216] RAX: 0000000000000000 RBX: ffff88810d56d1c0 RCX: ffff88810d56d1c0
[ 577.845860][ T7216] RDX: 0000000000000000 RSI: ffff88810d56d1c0 RDI: 0000000000000002
[ 577.856896][ T7216] RBP: ffff888025244700 R08: ffffffff8141a4ec R09: ffffed1004a488ed
[ 577.866712][ T7216] R10: ffff888025244763 R11: ffffed1004a488ec R12: ffff8880660b4c40
[ 577.875736][ T7216] R13: ffff888013930000 R14: ffff888025244700 R15: 0000000000000001
[ 577.889094][ T7216] ? __mmdrop+0x30c/0x400
[ 577.898466][ T7216] ? __mmdrop+0x30c/0x400
[ 577.907746][ T7216] finish_task_switch+0x56f/0x8c0
[ 577.917553][ T7216] ? __switch_to+0x580/0x1060
[ 577.926962][ T7216] __schedule+0xa04/0x2310
[ 577.937965][ T7216] ? firmware_map_remove+0x1a1/0x1a1

f2-vm login: [ 577.947035][ T7216] ? try_to_wake_up+0x7f3/0x16e0
[ 577.955799][ T7216] ? preempt_schedule_thunk+0x16/0x18
[ 577.964988][ T7216] preempt_schedule_common+0x4a/0xc0
[ 577.973670][ T7216] preempt_schedule_thunk+0x16/0x18
[ 577.985967][ T7216] try_to_wake_up+0x9eb/0x16e0
[ 577.994498][ T7216] ? migrate_swap_stop+0x9d0/0x9d0
[ 578.003265][ T7216] ? rcu_read_lock_held+0xae/0xc0
[ 578.012182][ T7216] ? rcu_read_lock_sched_held+0xe0/0xe0
[ 578.021280][ T7216] io_wqe_wake_worker.isra.0+0x4ba/0x670
[ 578.029857][ T7216] ? io_wq_manager+0xc00/0xc00
[ 578.041295][ T7216] ? _raw_spin_unlock_irqrestore+0x46/0x50
[ 578.050139][ T7216] io_wqe_enqueue+0x212/0x980
[ 578.058213][ T7216] __io_queue_async_work+0x201/0x4a0
[ 578.067518][ T7216] io_queue_async_work+0x52/0x80
[ 578.078327][ T7216] __io_queue_sqe+0x986/0xe80
[ 578.086615][ T7216] ? io_uring_setup+0x3a90/0x3a90
[ 578.094528][ T7216] ? radix_tree_load_root+0x119/0x1b0
[ 578.102598][ T7216] ? io_async_task_func+0xa90/0xa90
[ 578.110208][ T7216] ? __sanitizer_cov_trace_pc+0x1e/0x50
[ 578.120847][ T7216] io_queue_sqe+0x5e3/0xc40
[ 578.127950][ T7216] io_submit_sqes+0x17ca/0x26f0
[ 578.135559][ T7216] ? io_queue_sqe+0xc40/0xc40
[ 578.143129][ T7216] ? __x64_sys_io_uring_enter+0xa10/0xf00
[ 578.152183][ T7216] ? xa_store+0x40/0x50
[ 578.162501][ T7216] ? mutex_lock_io_nested+0x12a0/0x12a0
[ 578.170203][ T7216] ? do_raw_spin_unlock+0x175/0x260
[ 578.177874][ T7216] ? _raw_spin_unlock+0x28/0x40
[ 578.185560][ T7216] ? xa_store+0x40/0x50
[ 578.192755][ T7216] __x64_sys_io_uring_enter+0xa1b/0xf00
[ 578.201089][ T7216] ? __io_uring_task_cancel+0x1e0/0x1e0
[ 578.210378][ T7216] ? __sanitizer_cov_trace_pc+0x1e/0x50
[ 578.218401][ T7216] ? __audit_syscall_entry+0x3fe/0x540
[ 578.226264][ T7216] do_syscall_64+0x31/0x70
[ 578.234410][ T7216] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 578.244957][ T7216] RIP: 0033:0x7f5204b9c89d
[ 578.252372][ T7216] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c3 f5 0c 00 f7 d8 64 89 01 48
[ 578.272398][ T7216] RSP: 002b:00007ffd62bb14e8 EFLAGS: 00000212 ORIG_RAX: 00000000000001aa
[ 578.280966][ T7216] RAX: ffffffffffffffda RBX: 00007ffd62bb1560 RCX: 00007f5204b9c89d
[ 578.289068][ T7216] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000005
[ 578.300693][ T7216] RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000008
[ 578.308932][ T7216] R10: 0000000000000000 R11: 0000000000000212 R12: 0000000000000001
[ 578.317255][ T7216] R13: 0000000000000000 R14: 00007ffd62bb1520 R15: 0000000000000000
[ 578.328448][ T7216] Kernel Offset: disabled
[ 578.544329][ T7216] Rebooting in 86400 seconds..

The following program should be able to trigger the bug taken from my
xfstest-suite:

#include <liburing.h>

#define log_stderr(format, ...) \
fprintf(stderr, "%s: %d: %s - %m - " format "\n", __FILE__, __LINE__, __func__, \
##__VA_ARGS__);

#define log_error_errno(__ret__, __errno__, format, ...) \
({ \
typeof(__ret__) __internal_ret__ = (__ret__); \
errno = (__errno__); \
log_stderr(format, ##__VA_ARGS__); \
__internal_ret__; \
})


#define safe_close(fd) \
if (fd >= 0) { \
int _e_ = errno; \
close(fd); \
errno = _e_; \
fd = -EBADF; \
}

#define die_errno(__errno__, format, ...) \
({ \
errno = (__errno__); \
log_stderr(format, ##__VA_ARGS__); \
exit(EXIT_FAILURE); \
})

#define die(format, ...) die_errno(errno, format, ##__VA_ARGS__)

#define FILE1 "file1"

static int io_uring_openat_with_creds(struct io_uring *ring, int dfd, const char *path, int cred_id,
bool with_link)
{
struct io_uring_cqe *cqe;
struct io_uring_sqe *sqe;
int ret, i, to_submit = 1;

if (with_link) {
sqe = io_uring_get_sqe(ring);
io_uring_prep_nop(sqe);
sqe->flags |= IOSQE_IO_LINK;
sqe->user_data = 1;
to_submit++;
}

sqe = io_uring_get_sqe(ring);
io_uring_prep_openat(sqe, dfd, path, O_RDONLY | O_CLOEXEC, 0);
sqe->user_data = 2;

if (cred_id != -1)
sqe->personality = cred_id;

ret = io_uring_submit(ring);
if (ret != to_submit) {
log_stderr("failure: io_uring_submit");
goto out;
}

for (i = 0; i < to_submit; i++) {
ret = io_uring_wait_cqe(ring, &cqe);
if (ret < 0) {
log_stderr("failure: io_uring_wait_cqe");
goto out;
}

ret = cqe->res;
io_uring_cqe_seen(ring, cqe);
}
out:
return ret;
}

static inline bool switch_ids(uid_t uid, gid_t gid)
{
if (setgroups(0, NULL))
return log_errno(false, "failure: setgroups");

if (setresgid(gid, gid, gid))
return log_errno(false, "failure: setresgid");

if (setresuid(uid, uid, uid))
return log_errno(false, "failure: setresuid");

return true;
}

int wait_for_pid(pid_t pid)
{
int status, ret;

again:
ret = waitpid(pid, &status, 0);
if (ret == -1) {
if (errno == EINTR)
goto again;

return -1;
}

if (!WIFEXITED(status))
return -1;

return WEXITSTATUS(status);
}

static int io_uring(void)
{
int fret = -1;
int file1_fd = -EBADF;
struct io_uring ring = {};
int cred_id, ret;
pid_t pid;

ret = io_uring_queue_init(8, &ring, 0);
if (ret)
return log_error_errno(-1, -ret, "failure: io_uring_queue_init");

ret = io_uring_register_personality(&ring);
if (ret < 0)
return 0; /* personalities not supported */
cred_id = ret;

/* create file only owner can open */
file1_fd = openat(t_dir1_fd, FILE1, O_RDONLY | O_CREAT | O_EXCL | O_CLOEXEC, 0000);
if (file1_fd < 0) {
log_stderr("failure: openat");
goto out;
}
if (fchown(file1_fd, 0, 0)) {
log_stderr("failure: fchown");
goto out;
}
if (fchmod(file1_fd, 0600)) {
log_stderr("failure: fchmod");
goto out;
}
safe_close(file1_fd);

pid = fork();
if (pid < 0) {
log_stderr("failure: fork");
goto out;
}
if (pid == 0) {
/* Verify we can open it with our current credentials. */
file1_fd = io_uring_openat_with_creds(&ring, t_dir1_fd, FILE1, -1, false);
if (file1_fd < 0)
die("failure: io_uring_open_file");

exit(EXIT_SUCCESS);
}
if (wait_for_pid(pid)) {
log_stderr("failure: wait_for_pid");
goto out;
}

pid = fork();
if (pid < 0) {
log_stderr("failure: fork");
goto out;
}
if (pid == 0) {
if (!switch_ids(1000, 1000))
die("failure: switch_ids");

/* Verify we can't open it with our current credentials. */
file1_fd = io_uring_openat_with_creds(&ring, t_dir1_fd, FILE1, -1, false);
if (file1_fd >= 0)
die("failure: io_uring_open_file");

exit(EXIT_SUCCESS);
}
if (wait_for_pid(pid)) {
log_stderr("failure: wait_for_pid");
goto out;
}

pid = fork();
if (pid < 0) {
log_stderr("failure: fork");
goto out;
}
if (pid == 0) {
if (!switch_ids(1000, 1000))
die("failure: switch_ids");

/* Verify we can open it with the registered credentials. */
file1_fd = io_uring_openat_with_creds(&ring, t_dir1_fd, FILE1, cred_id, false);
if (file1_fd < 0)
die("failure: io_uring_open_file");

/*
* Verify we can open it with the registered credentials and as
* a link.
*/
file1_fd = io_uring_openat_with_creds(&ring, t_dir1_fd, FILE1, cred_id, true);
if (file1_fd < 0)
die("failure: io_uring_open_file");

exit(EXIT_SUCCESS);
}
if (wait_for_pid(pid)) {
log_stderr("failure: wait_for_pid");
goto out;
}

fret = 0;
out:
ret = io_uring_unregister_personality(&ring, cred_id);
if (ret)
log_stderr("failure: io_uring_unregister_personality");

safe_close(file1_fd);

return fret;
}

Thanks!
Christian