[BUG] misc/mei: Race between mei_release() disconnect and mei_ioctl_connect_vtag() causes CSME reset storm and i915 freeze
From: nirbhayykumarr
Date: Sat Aug 29 2026 - 03:14:40 EST
Hi all,
This issue was discovered using a custom multi-threaded C fuzzer
designed to stress-test MEI Virtual Tag (vtag) client lifecycles and
multiplexing over /dev/mei0. By concurrently racing rapid vtag
connections against file descriptor closures and streaming I/O, a
race condition is triggered during client teardown.
System Information & Environment:
- Kernel Version: 7.1.8-zen1-3-zen x86_64 (drivers/misc/mei is identical to upstream)
- Kernel Taint: 0 (Not tainted)
- CPU / Platform: 12th Gen Intel(R) Core(TM) i5-12500H (Alder Lake-P)
- MEI Controller: 00:16.0 Intel Alder Lake PCH HECI Controller (/dev/mei0)
- Graphics / DRM: Intel Iris Xe Graphics (i915 driver with mei_hdcp and mei_pxp components)
- Subsystems: drivers/misc/mei, drivers/gpu/drm/i915
In mei_release(), closing the last file descriptor holding a virtual tag
invokes mei_cl_disconnect(). Inside __mei_cl_disconnect(), dev->device_lock
is dropped while awaiting the firmware disconnect ACK on cl->wait.
During this lock-drop window, a concurrent IOCTL_MEI_CONNECT_CLIENT_VTAG
call on the same UUID (e.g. MKHI) scans dev->file_list, matches the tearing-
down client 'pos' (in MEI_FILE_DISCONNECTING), repoints file->private_data
to pos, and adds its new vtag to pos->vtag_map.
When the disconnect ACK arrives, __mei_cl_disconnect() calls
mei_cl_set_disconnected(cl), setting cl->me_cl = NULL and
cl->state = MEI_FILE_DISCONNECTED. Because pos->vtag_map now contains the
second thread's tag, mei_release() skips unlinking/freeing cl. The second
thread then wakes up and attempts to reconnect via mei_ioctl_connect_client().
Additionally, shared clients lack tag based demuxing on cl->rd_pending in
drivers/misc/mei/interrupt.c:
cb = list_first_entry_or_null(&cl->rd_pending, struct mei_cl_cb, list);
Incoming packets are matched to the head of the FIFO queue regardless of
vtag, causing -EPROTO errors, dropped packets, and out-of-order HBM command
sequences. This triggers continuous CSME hardware link resets:
mei mei0: FW not ready: resetting: dev_state = 3
mei mei0: unexpected reset: dev_state = ENABLED fw status = ...
During each reset, child client drivers (mei_hdcp, mei_pxp) unbind and rebind
with i915 DRM. With resets looping at hundreds of cycles per second (>9,800
events in 27s), mei_cldev_enable() repeatedly fails with -EFAULT / -ENODEV,
deadlocking i915 display worker mutexes in TASK_UNINTERRUPTIBLE and causing
an unrecoverable full system freeze.
Journalctl logs:
mei mei0: FW not ready: resetting: dev_state = 3
mei mei0: unexpected reset: dev_state = ENABLED fw status = 90000245 89110106 00000020 00004000 00021F03 446003CB
mei_hdcp 0000:00:16.0-b638ab7e-94e2-4ea2-a552-d1c54b627f04: mei_cldev_enable Failed. -14
mei_hdcp 0000:00:16.0-b638ab7e-94e2-4ea2-a552-d1c54b627f04: probe with driver mei_hdcp failed with error -14
mei_hdcp 0000:00:16.0-b638ab7e-94e2-4ea2-a552-d1c54b627f04: bound 0000:00:02.0 (ops i915_hdcp_ops [i915])
mei_pxp 0000:00:16.0-fbf6fcf1-96cf-4e2e-a6a6-1bab8cbe36b1: bound 0000:00:02.0 (ops i915_pxp_tee_component_ops [i915])
mei mei0: FW not ready: resetting: dev_state = 3
mei mei0: unexpected reset: dev_state = ENABLED fw status = 90000245 89110106 00000020 00004000 00021F03 446003CB
Proposed Fix:
- Prevent vtag reuse during teardown: in mei_ioctl_connect_vtag(),
ignore existing clients on dev->file_list if they are in
MEI_FILE_DISCONNECTING or MEI_FILE_DISCONNECTED states or undergoing
teardown.
- Implement proper reference counting / lifecycle synchronization on
shared struct mei_cl instances.
- In drivers/misc/mei/interrupt.c, demux cl->rd_pending by matching the
incoming packet's vtag header to the corresponding callback rather
than assuming FIFO order.
- Add rate-limiting / backoff to MEI client reprobing during hardware
link resets to prevent cascading bus storms into DRM / i915.
A minimal standalone C reproducer (repro_mei_vtag.c) is attached below.
WARNING: Running this reproducer triggers an unrecoverable hard freeze
and display deadlock requiring a forced restart.
===================================================================
Standalone C Reproducer (repro_mei_vtag.c)
===================================================================
/*
* repro_mei_vtag.c - Minimal Standalone Reproducer for MEI Vtag Reconnect Race
* Compilation: gcc -Wall -Wextra -O2 -pthread repro_mei_vtag.c -o repro_mei_vtag
* Execution: sudo ./repro_mei_vtag /dev/mei0
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <pthread.h>
#include <signal.h>
#include <stdatomic.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <unistd.h>
#pragma pack(push, 1)
typedef struct {
uint8_t b[16];
} uuid_le;
struct mei_client {
uint32_t max_msg_length;
uint8_t protocol_version;
uint8_t reserved[3];
};
struct mei_connect_client_data_vtag {
union {
struct {
uuid_le in_client_uuid;
uint8_t vtag;
uint8_t reserved[3];
} connect;
struct mei_client out_client_properties;
};
};
#pragma pack(pop)
#ifndef IOCTL_MEI_CONNECT_CLIENT_VTAG
#define IOCTL_MEI_CONNECT_CLIENT_VTAG \
_IOWR('H', 0x04, struct mei_connect_client_data_vtag)
#endif
static const uuid_le MKHI_UUID = {.b = {0x15, 0x67, 0x6a, 0x8e, 0xbc, 0x9a,
0x43, 0x40, 0x88, 0xef, 0x9e, 0x39,
0xc6, 0xf6, 0x3e, 0x0f}};
static volatile sig_atomic_t g_running = 1;
static void sig_handler(int sig) { (void)sig; g_running = 0; }
static atomic_uint_fast64_t g_t1_ops = 0;
static atomic_uint_fast64_t g_t2_ops = 0;
static atomic_uint_fast64_t g_t3_ops = 0;
static int connect_vtag(const char *dev_path, uint8_t vtag) {
int fd = open(dev_path, O_RDWR | O_NONBLOCK);
if (fd < 0)
return -1;
struct mei_connect_client_data_vtag data;
memset(&data, 0, sizeof(data));
data.connect.in_client_uuid = MKHI_UUID;
data.connect.vtag = vtag;
if (ioctl(fd, IOCTL_MEI_CONNECT_CLIENT_VTAG, &data) < 0) {
close(fd);
return -1;
}
return fd;
}
static void *thread_vtag1(void *arg) {
const char *dev = (const char *)arg;
uint8_t cmd[4] = {0x07, 0x01, 0x00, 0x00};
while (g_running) {
int fd = connect_vtag(dev, 1);
if (fd >= 0) {
atomic_fetch_add(&g_t1_ops, 1);
write(fd, cmd, sizeof(cmd));
usleep(rand() % 40);
close(fd);
} else {
usleep(50);
}
}
return NULL;
}
static void *thread_vtag2(void *arg) {
const char *dev = (const char *)arg;
uint8_t cmd[4] = {0x07, 0x02, 0x00, 0x00};
while (g_running) {
int fd = connect_vtag(dev, 2);
if (fd >= 0) {
atomic_fetch_add(&g_t2_ops, 1);
write(fd, cmd, sizeof(cmd));
usleep(rand() % 60);
close(fd);
} else {
usleep(50);
}
}
return NULL;
}
static void *thread_vtag3(void *arg) {
const char *dev = (const char *)arg;
uint8_t cmd[4] = {0x07, 0x01, 0x00, 0x00};
uint8_t buf[128];
while (g_running) {
int fd = connect_vtag(dev, 3);
if (fd >= 0) {
struct pollfd pfd = {.fd = fd, .events = POLLIN};
for (int i = 0; i < 15 && g_running; i++) {
if (write(fd, cmd, sizeof(cmd)) > 0) {
atomic_fetch_add(&g_t3_ops, 1);
if (poll(&pfd, 1, 50) > 0)
read(fd, buf, sizeof(buf));
}
usleep(300);
}
close(fd);
} else {
usleep(500);
}
}
return NULL;
}
int main(int argc, char **argv) {
const char *dev = (argc > 1) ? argv[1] : "/dev/mei0";
signal(SIGINT, sig_handler);
signal(SIGTERM, sig_handler);
int test_fd = connect_vtag(dev, 1);
if (test_fd < 0) {
fprintf(stderr, "[-] Failed to connect with vtag on %s: %s\n", dev, strerror(errno));
return 1;
}
close(test_fd);
pthread_t t1, t2, t3;
pthread_create(&t1, NULL, thread_vtag1, (void *)dev);
pthread_create(&t2, NULL, thread_vtag2, (void *)dev);
pthread_create(&t3, NULL, thread_vtag3, (void *)dev);
while (g_running) {
printf("\r[+] T1 (vtag=1 close): %lu | T2 (vtag=2 race): %lu | T3 (vtag=3 io): %lu",
atomic_load(&g_t1_ops), atomic_load(&g_t2_ops), atomic_load(&g_t3_ops));
fflush(stdout);
usleep(250000);
}
pthread_join(t1, NULL);
pthread_join(t2, NULL);
pthread_join(t3, NULL);
return 0;
}
===================================================================
Best regards,
Nirbhay Kumar