[PATCH 5.4 15/68] platform/chrome: cros_ec_debugfs: detach log reader wq from devm

Greg Kroah-Hartman
2022-05-23T17:28:19Z

From: Tzung-Bi Shih <tzungbi@google.com>

[ Upstream commit 0e8eb5e8acbad19ac2e1856b2fb2320184299b33 ]

Debugfs console_log uses devm memory (e.g. debug_info in
cros_ec_console_log_poll()). However, lifecycles of device and debugfs
are independent. An use-after-free issue is observed if userland
program operates the debugfs after the memory has been freed.

The call trace:
do_raw_spin_lock
_raw_spin_lock_irqsave
remove_wait_queue
ep_unregister_pollwait
ep_remove
do_epoll_ctl

A Python example to reproduce the issue:
... import select
... p = select.epoll()
... f = open('/sys/kernel/debug/cros_scp/console_log')
... p.register(f, select.POLLIN)
... p.poll(1)
[(4, 1)] # 4=fd, 1=select.POLLIN

[ shutdown cros_scp at the point ]

... p.poll(1)
[(4, 16)] # 4=fd, 16=select.POLLHUP
... p.unregister(f)

An use-after-free issue raises here. It called epoll_ctl with
EPOLL_CTL_DEL which in turn to use the workqueue in the devm (i.e.
log_wq).

Detaches log reader's workqueue from devm to make sure it is persistent
even if the device has been removed.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Reviewed-by: Guenter Roeck <groeck@google.com>
Link: https://lore.kernel.org/r/20220209051130.386175-1-tzungbi@google.com
Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/platform/chrome/cros_ec_debugfs.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/platform/chrome/cros_ec_debugfs.c b/drivers/platform/chrome/cros_ec_debugfs.c
index 6ae484989d1f..c4b57e1df192 100644
--- a/drivers/platform/chrome/cros_ec_debugfs.c
+++ b/drivers/platform/chrome/cros_ec_debugfs.c
@@ -26,6 +26,9 @@

#define CIRC_ADD(idx, size, value) (((idx) + (value)) & ((size) - 1))

+/* waitqueue for log readers */
+static DECLARE_WAIT_QUEUE_HEAD(cros_ec_debugfs_log_wq);
+
/**
* struct cros_ec_debugfs - EC debugging information.
*
@@ -34,7 +37,6 @@
* @log_buffer: circular buffer for console log information
* @read_msg: preallocated EC command and buffer to read console log
* @log_mutex: mutex to protect circular buffer
- * @log_wq: waitqueue for log readers
* @log_poll_work: recurring task to poll EC for new console log data
* @panicinfo_blob: panicinfo debugfs blob
*/
@@ -45,7 +47,6 @@ struct cros_ec_debugfs {
struct circ_buf log_buffer;
struct cros_ec_command *read_msg;
struct mutex log_mutex;
- wait_queue_head_t log_wq;
struct delayed_work log_poll_work;
/* EC panicinfo */
struct debugfs_blob_wrapper panicinfo_blob;
@@ -108,7 +109,7 @@ static void cros_ec_console_log_work(struct work_struct *__work)
buf_space--;
}

- wake_up(&debug_info->log_wq);
+ wake_up(&cros_ec_debugfs_log_wq);
}

mutex_unlock(&debug_info->log_mutex);
@@ -142,7 +143,7 @@ static ssize_t cros_ec_console_log_read(struct file *file, char __user *buf,

mutex_unlock(&debug_info->log_mutex);

- ret = wait_event_interruptible(debug_info->log_wq,
+ ret = wait_event_interruptible(cros_ec_debugfs_log_wq,
CIRC_CNT(cb->head, cb->tail, LOG_SIZE));
if (ret < 0)
return ret;
@@ -174,7 +175,7 @@ static __poll_t cros_ec_console_log_poll(struct file *file,
struct cros_ec_debugfs *debug_info = file->private_data;
__poll_t mask = 0;

- poll_wait(file, &debug_info->log_wq, wait);
+ poll_wait(file, &cros_ec_debugfs_log_wq, wait);

mutex_lock(&debug_info->log_mutex);
if (CIRC_CNT(debug_info->log_buffer.head,
@@ -359,7 +360,6 @@ static int cros_ec_create_console_log(struct cros_ec_debugfs *debug_info)
debug_info->log_buffer.tail = 0;

mutex_init(&debug_info->log_mutex);
- init_waitqueue_head(&debug_info->log_wq);

debugfs_create_file("console_log", S_IFREG | 0444, debug_info->dir,
debug_info, &cros_ec_console_log_fops);
--
2.35.1