linux-kernel - Re: [PATCH] tty: vt: check for atomic context in con

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Date: Mon, 22 Jan 2024 23:08:29 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Jiri Slaby <jirislaby@...nel.org>,
        syzbot <syzbot+06fa1063cca8163ea541@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Starke, Daniel" <daniel.starke@...mens.com>,
        Lee Jones <lee@...nel.org>, Fedor Pchelkin <pchelkin@...ras.ru>
Cc: linux-kernel@...r.kernel.org, linux-serial <linux-serial@...r.kernel.org>
Subject: Re: [PATCH] tty: vt: check for atomic context in con_write()

On 2024/01/22 15:48, Jiri Slaby wrote:
> On 20. 01. 24, 11:34, Tetsuo Handa wrote:
>> syzbot is reporting sleep in atomic context, for gsmld_write() is calling
>> con_write() with spinlock held and IRQs disabled.
> 
> gsm should never be bound to a console in the first place.
> 
> Noone has sent a patch to deny that yet.
> 
> Follow:
> https://lore.kernel.org/all/49453ebd-b321-4f34-a1a5-d828d8881010@kernel.org/
> 
> And feel free to patch that ;).
> 
> thanks,

OK. Here is a deny-listing based filter using device number of sysfs entry.
(We don't want to compare with the function address of con_write().
Thus, this patch is comparing with device major/minor numbers.)

----------
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 4036566febcb..6f9730dce5aa 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3630,6 +3630,10 @@ static int gsmld_open(struct tty_struct *tty)
 	if (tty->ops->write == NULL)
 		return -EINVAL;
 
+	/* Can't be attached to virtual consoles. */
+	if (tty->dev && MAJOR(tty->dev->devt) == 4 && MINOR(tty->dev->devt) < 64)
+		return -EINVAL;
+
 	/* Attach our ldisc data */
 	gsm = gsm_alloc_mux();
 	if (gsm == NULL)
----------

Is it possible to use allow-listing based filtering?
(Attaching on /dev/tty (major=5, minor=0) causes current ssh session
to be closed. Unexpectedly loosing connection might be a problem for
fuzz testing...)

----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/tty.h>

int main(int argc, char *argv[]) {
        int ldisc = N_GSM0710;

        return ioctl(open(argv[1], O_RDWR | O_NOCTTY | O_NDELAY), TIOCSETD, &ldisc) == 0;
}
----------