Messages in this thread | | | From | "Fabio M. De Francesco" <> | Subject | Re: [PATCH] vt: Fix sleeping functions called from atomic context | Date | Wed, 17 Nov 2021 13:38:33 +0100 |
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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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