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SubjectRe: [PATCH] vt: Fix sleeping functions called from atomic context
Date
On Wednesday, November 17, 2021 9:54:59 AM CET Greg Kroah-Hartman wrote:
> On Wed, Nov 17, 2021 at 09:23:45AM +0100, Fabio M. De Francesco wrote:
> > On Tuesday, November 16, 2021 5:59:24 PM CET Greg Kroah-Hartman wrote:
> > > On Tue, Nov 16, 2021 at 04:35:07PM +0100, Fabio M. De Francesco wrote:
> > > > On Tuesday, November 16, 2021 3:58:44 PM CET Greg Kroah-Hartman
wrote:
> > > > > On Tue, Nov 16, 2021 at 03:49:37PM +0100, Fabio M. De Francesco
wrote:
> > > > > > Fix two sleeping functions called from atomic context by doing
> > immediate
> > > > > > return to the caller if !preemptible() evaluates 'true'. Remove
two
> > > > > > in_interrupt() tests because they are not suited for being used
here.
> > > > > >
> > > > > > Since functions do_con_write() and con_flush_chars() might sleep
in
> > > > > > console_lock(), it must be assured that they are never executed
in
> > > > > > atomic contexts.
> > > > > >
> > > > > > This issue is reported by Syzbot which notices that they are
executed
> > > > > > while holding spinlocks and with interrupts disabled. Actually
Syzbot
> > > > > > emits a first report and then, after fixing do_con_write(), a
second
> > > > > > report for the same problem in con_flush_chars() because these
> > functions
> > > > > > are called one after the other by con_write().
> > > > > >
> > > > > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > > > > Reported-by:
syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com
> > > > > > Suggested-by: Marco Elver <elver@google.com>
> > > > > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > > > > > ---
> > > > > > drivers/tty/vt/vt.c | 4 ++--
> > > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > > > > > index 7359c3e80d63..508f8a56d361 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 (!preemptible())
> > > > > > return count;
> > > > >
> > > > > Very odd, what code is calling these functions to trigger this
check?
> > > >
> > > > This is the call trace reported by Syzbot (https://
syzkaller.appspot.com/
> > bug?
> > > > id=fe5a4d5a2482bd73064db5de5d28e024f1e2a387):
> > > >
> > > > 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:9539
> > > > console_lock+0x17/0x80 kernel/printk/printk.c:2522
> > > > do_con_write+0x10f/0x1e40 drivers/tty/vt/vt.c:2908
> > > > con_write+0x21/0x40 drivers/tty/vt/vt.c:3295
> > > > n_hdlc_send_frames+0x24b/0x490 drivers/tty/n_hdlc.c:290
> > > > tty_wakeup+0xe1/0x120 drivers/tty/tty_io.c:534
> > > > __start_tty drivers/tty/tty_io.c:806 [inline]
> > > > __start_tty+0xfb/0x130 drivers/tty/tty_io.c:799
> > > > n_tty_ioctl_helper+0x299/0x2d0 drivers/tty/tty_ioctl.c:880
> > > >
> > > > ^^^^^^^^^^
> > > > n_tty_ioctl_helper() disabled interrupts via spin_lock_irq(&tty-
> > >flow.lock).
> > > >
> > > > n_hdlc_tty_ioctl+0xd2/0x340 drivers/tty/n_hdlc.c:633
> > > > tty_ioctl+0xc69/0x1670 drivers/tty/tty_io.c:2814
> > > > vfs_ioctl fs/ioctl.c:51 [inline]
> > > > __do_sys_ioctl fs/ioctl.c:874 [inline]
> > > > __se_sys_ioctl fs/ioctl.c:860 [inline]
> > > > __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
> > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > > > entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > >
> > > > > Shouldn't the caller be fixed instead?
> > > >
> > > > Maybe that the caller has no need to disable IRQs, but I cannot yet
> > answer to
> > > > this particular question.
> > > >
> > > > > What changed to suddenly cause this to show up?
> > > >
> > > > Commit c545b66c6922 ("tty: Serialize tcflow() with other tty flow
control
> > > > changes") introduced a call to spin_lock_irq() for command "TCOON",
just
> > > > before calling __start_tty().
> > >
> > > That commit happened in 2014. Why is this suddenly an issue now that
no
> > > one ever saw before?
> >
> > Hi Greg,
> >
> > I have just been informed by Pavel Skripkin (who at this moment cannot
> > directly write to LKML) that this bug has been reported at least other
two
> > times.
> >
> > We cannot say why nobody has ever addressed this issue before.
> >
> > Please read the following reports.
> >
> > https://syzkaller.appspot.com/bug?
id=9ed44c22919e3cd2c71907b963fb048ac1115073
> > https://syzkaller.appspot.com/bug?
id=e806305a3a65a0bb2f1be7e672819a079d558016
>
> Great, you have a reproducer, so you should be able to duplicate this
> locally to figure out what is really happening here.
>
> Good luck!
>
> greg k-h
>
Hi Greg,

As you asked for, I've run the reproducer (as root) while "trace-cmd record -
g n_tty_ioctl_helper -p function_graph sleep 15" was in execution.

The call trace from n_tty_ioctl_helper() to console_lock() has been recorded
and for sure console_lock() is called while holding a spinlock and with IRQs
disabled via spin_lock_irq().

The following is the relevant part of "trace-cmd report"...

repro-27397 [004] 18814.535029: funcgraph_entry: |
n_tty_ioctl_helper() {
repro-27397 [004] 18814.535030: funcgraph_entry: 0.676 us
| tty_check_change();
repro-27397 [004] 18814.535032: funcgraph_entry: 0.761 us
| _raw_spin_lock_irq();
repro-27397 [004] 18814.535033: funcgraph_entry:
| __start_tty() {
repro-27397 [004] 18814.535034: funcgraph_entry:
| con_start() {

---- cut not relevant code ----

repro-27397 [004] 18814.535088: funcgraph_entry: |
con_write() {
repro-27397 [004] 18814.535088: funcgraph_entry:
| do_con_write() {
repro-27397 [004] 18814.535088: funcgraph_entry:
| console_lock() {
repro-27397 [004] 18814.535089: funcgraph_entry:
| __cond_resched() {

---- cut not relevant code ----

repro-27397 [004] 18814.571839: funcgraph_entry: 0.641 us |
__printk_safe_exit();
repro-27397 [004] 18814.571841: funcgraph_exit: + 18.902 us
| }
repro-27397 [004] 18814.571842: funcgraph_exit: + 31.651 us
| }
repro-27397 [004] 18814.571843: funcgraph_exit: # 36754.817
us | }

Please notice that "#36754.817 us" when finally console_lock() returned.

May very well be that I've overlooked something (as you know I'm a newbie and
all this is quite new for me :)

As I wrote (please read it again above) for now I confirm that commit
c545b66c6922 ("tty: Serialize tcflow() with other tty flow control changes")
introduced a call to spin_lock_irq() for command "TCOON".

However I have to double check it.

If it is confirmed, I will reply soon to Tetsuo's message because he says
that the relevant commit should be a different one.

However, what is relevant here is that for sure console_lock() is called in
atomic context and that the Syzbot report is not a false positive. Am I
missing something?

About the possible fixes I only see two potential solutions:

1) If we need for real spin_lock_irq() in n_tty_ioctl_helper() we should test
the context before calling console_lock() (it is annotated with
might_sleep()).

2) If we don't need to disable IRQs we should fix n_tty_ioctl_helper().

I'm not able to go deeper and understand whether or not that spin_lock_irq()
is an overkill and if it can be converted to a different kind of locking API.

I'd appreciate feedback from you and other experienced developers.

Thank you very much,

Fabio M. De Francesco





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