Re: [syzbot] BUG: sleeping function called from invalid context in __might_resched

[Fabio M. De Francesco]

On Tuesday, November 16, 2021 9:53:53 AM CET Fabio M. De Francesco wrote:
> On Tuesday, November 16, 2021 9:09:11 AM CET syzbot wrote:
> > Hello,
> > 
> > syzbot has tested the proposed patch but the reproducer is still 
triggering 
> an issue:
> > BUG: sleeping function called from invalid context in __might_resched
> > 
> > BUG: sleeping function called from invalid context at kernel/printk/
> printk.c:2522
> > in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 8755, name: syz-
> executor.2
> > preempt_count: 1, expected: 0
> > RCU nest depth: 0, expected: 0
> > 3 locks held by syz-executor.2/8755:
> >  #0: ffff888070c9a098
> >  (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/
> tty/tty_ldisc.c:252
> >  #1: ffff888070c9a468
> >  (&tty->flow.lock){....}-{2:2}, at: spin_lock_irq include/linux/
spinlock.h:
> 374 [inline]
> >  (&tty->flow.lock){....}-{2:2}, at: n_tty_ioctl_helper+0xb6/0x2d0 
drivers/
> tty/tty_ioctl.c:877
> >  #2: ffff888070c9a098 (&tty->ldisc_sem){++++}-{0:0}, at: 
> tty_ldisc_ref+0x1d/0x80 drivers/tty/tty_ldisc.c:273
> > irq event stamp: 916
> > hardirqs last  enabled at (915): [<ffffffff81beabd5>] 
> kasan_quarantine_put+0xf5/0x210 mm/kasan/quarantine.c:220
> > hardirqs last disabled at (916): [<ffffffff8950a731>] __raw_spin_lock_irq 
> include/linux/spinlock_api_smp.h:117 [inline]
> > hardirqs last disabled at (916): [<ffffffff8950a731>] 
> _raw_spin_lock_irq+0x41/0x50 kernel/locking/spinlock.c:170
> > softirqs last  enabled at (0): [<ffffffff8144cf0c>] copy_process+0x1e8c/
> 0x75a0 kernel/fork.c:2136
> > softirqs last disabled at (0): [<0000000000000000>] 0x0
> > Preemption disabled at:
> > [<0000000000000000>] 0x0
> > CPU: 1 PID: 8755 Comm: syz-executor.2 Not tainted 5.16.0-rc1-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS 
> Google 01/01/2011
> > Call Trace:
> >  <TASK>
> >  __dump_stack lib/dump_stack.c:88 [inline]
> >  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> >  __might_resched.cold+0x222/0x26b kernel/sched/core.c:9542
> >  console_lock+0x17/0x80 kernel/printk/printk.c:2522
> >  con_flush_chars drivers/tty/vt/vt.c:3365 [inline]
> >  con_flush_chars+0x35/0x90 drivers/tty/vt/vt.c:3357
> >  con_write+0x2c/0x40 drivers/tty/vt/vt.c:3296
> 
> The reproducer is still triggering an issue, but this time it looks like it 
> is triggered by a different path of execution.
> 
> The same invalid "in_interrupt()" test is also in con_flush_chars().
> 
> Let's try to remove it too...
> 
> My first idea would be to replace "if (in_interrupt())" with the same 
> "preempt_count() || irqs_disabled()" I used in do_con_write(). However I 
> noticed that both do_con_write() and con_flush_chars() are only called from 
> inside con_write() (which, aside from calling those functions, does nothing 
> else).
> 
> So why not remove the if (in_interrupt()) from both them and use if 
> (preempt_count() || irqs_disabled()) just once in con_write()?
> 
> I think this should be the right solution, but I prefer to go one step at a 
> time.
> 
> Therefore, I'll (1) use the same (redundant, if it was used in con_write()) 
> test also in con_flush_chars(), (2) wait for Syzbot to confirm that it 
fixes 
> the bug, and (3) wait for maintainers review and suggestions about whether 
or 
> not moving those tests one level upper.
> 

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

> 
> ---
> Fabio M. De Francesco
> 

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 7359c3e80d63..46511d1ac6ee 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -2902,7 +2902,7 @@ static int do_con_write(struct tty_struct *tty, const unsigned char *buf, int co
 	struct vt_notifier_param param;
 	bool rescan;
 
-	if (in_interrupt())
+	if (preempt_count() || irqs_disabled())
 		return count;
 
 	console_lock();
@@ -3358,7 +3358,7 @@ static void con_flush_chars(struct tty_struct *tty)
 {
 	struct vc_data *vc;
 
-	if (in_interrupt())	/* from flush_to_ldisc */
+	if (preempt_count() || irqs_disabled())	/* from flush_to_ldisc */
 		return;
 
 	/* if we race with con_close(), vt may be null */