[PATCH 5.15 116/177] tty: n_hdlc: make n_hdlc_tty_wakeup() asynchronous

Greg Kroah-Hartman
2021-12-20T15:14:03Z

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

commit 1ee33b1ca2b8dabfcc17198ffd049a6b55674a86 upstream.

syzbot is reporting that an unprivileged user who logged in from tty
console can crash the system using a reproducer shown below [1], for
n_hdlc_tty_wakeup() is synchronously calling n_hdlc_send_frames().

----------
#include <sys/ioctl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
const int disc = 0xd;

ioctl(1, TIOCSETD, &disc);
while (1) {
ioctl(1, TCXONC, 0);
write(1, "", 1);
ioctl(1, TCXONC, 1); /* Kernel panic - not syncing: scheduling while atomic */
}
}
----------

Linus suspected that "struct tty_ldisc"->ops->write_wakeup() must not
sleep, and Jiri confirmed it from include/linux/tty_ldisc.h. Thus, defer
n_hdlc_send_frames() from n_hdlc_tty_wakeup() to a WQ context like
net/nfc/nci/uart.c does.

Link: https://syzkaller.appspot.com/bug?extid=5f47a8cea6a12b77a876 [1]
Reported-by: syzbot <syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Analyzed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Confirmed-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: https://lore.kernel.org/r/40de8b7e-a3be-4486-4e33-1b1d1da452f8@i-love.sakura.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/n_hdlc.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -140,6 +140,8 @@ struct n_hdlc {
struct n_hdlc_buf_list rx_buf_list;
struct n_hdlc_buf_list tx_free_buf_list;
struct n_hdlc_buf_list rx_free_buf_list;
+ struct work_struct write_work;
+ struct tty_struct *tty_for_write_work;
};

/*
@@ -154,6 +156,7 @@ static struct n_hdlc_buf *n_hdlc_buf_get
/* Local functions */

static struct n_hdlc *n_hdlc_alloc(void);
+static void n_hdlc_tty_write_work(struct work_struct *work);

/* max frame size for memory allocations */
static int maxframe = 4096;
@@ -210,6 +213,8 @@ static void n_hdlc_tty_close(struct tty_
wake_up_interruptible(&tty->read_wait);
wake_up_interruptible(&tty->write_wait);

+ cancel_work_sync(&n_hdlc->write_work);
+
n_hdlc_free_buf_list(&n_hdlc->rx_free_buf_list);
n_hdlc_free_buf_list(&n_hdlc->tx_free_buf_list);
n_hdlc_free_buf_list(&n_hdlc->rx_buf_list);
@@ -241,6 +246,8 @@ static int n_hdlc_tty_open(struct tty_st
return -ENFILE;
}

+ INIT_WORK(&n_hdlc->write_work, n_hdlc_tty_write_work);
+ n_hdlc->tty_for_write_work = tty;
tty->disc_data = n_hdlc;
tty->receive_room = 65536;

@@ -335,6 +342,20 @@ check_again:
} /* end of n_hdlc_send_frames() */

/**
+ * n_hdlc_tty_write_work - Asynchronous callback for transmit wakeup
+ * @work: pointer to work_struct
+ *
+ * Called when low level device driver can accept more send data.
+ */
+static void n_hdlc_tty_write_work(struct work_struct *work)
+{
+ struct n_hdlc *n_hdlc = container_of(work, struct n_hdlc, write_work);
+ struct tty_struct *tty = n_hdlc->tty_for_write_work;
+
+ n_hdlc_send_frames(n_hdlc, tty);
+} /* end of n_hdlc_tty_write_work() */
+
+/**
* n_hdlc_tty_wakeup - Callback for transmit wakeup
* @tty: pointer to associated tty instance data
*
@@ -344,7 +365,7 @@ static void n_hdlc_tty_wakeup(struct tty
{
struct n_hdlc *n_hdlc = tty->disc_data;

- n_hdlc_send_frames(n_hdlc, tty);
+ schedule_work(&n_hdlc->write_work);
} /* end of n_hdlc_tty_wakeup() */

/**