Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic - not syncing: panic_on_warn set ..."

From: Dmitry Vyukov
Date: Mon Mar 18 2019 - 08:39:23 EST

On Mon, Mar 18, 2019 at 1:32 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Mon, Mar 18, 2019 at 1:07 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >
> > On Mon, Mar 18, 2019 at 6:27 AM Tetsuo Handa
> > <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Dmitry Vyukov wrote:
> > > > > Then, we need to find what test is changing console_loglevel.
> > > > > Maybe add debug BUG_ON() in linux-next.git using CONFIG_DEBUG_AID_FOR_SYZBOT ?
> > > >
> > > > Is there a single place to catch this? I could run syzkaller locally
> > > > first with the check.
> > > >
> > >
> > > There is no such place. But not so many places change permanently.
> > > For x86, you can test with below patch applied.
> > >
> > > ---
> > > drivers/tty/sysrq.c | 1 +
> > > kernel/printk/printk.c | 3 +++
> > > kernel/sysctl.c | 23 ++++++++++++++++++++++-
> > > 3 files changed, 26 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> > > index fa0ce7d..ad73520 100644
> > > --- a/drivers/tty/sysrq.c
> > > +++ b/drivers/tty/sysrq.c
> > > @@ -91,6 +91,7 @@ static void sysrq_handle_loglevel(int key)
> > > i = key - '0';
> > > console_loglevel = CONSOLE_LOGLEVEL_DEFAULT;
> > > pr_info("Loglevel set to %d\n", i);
> > > + WARN_ONCE(i < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", i);
> > > console_loglevel = i;
> > > }
> > > static struct sysrq_key_op sysrq_loglevel_op = {
> > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > > index 02ca827..70d1f8c 100644
> > > --- a/kernel/printk/printk.c
> > > +++ b/kernel/printk/printk.c
> > > @@ -1533,11 +1533,13 @@ int do_syslog(int type, char __user *buf, int len, int source)
> > > case SYSLOG_ACTION_CONSOLE_OFF:
> > > if (saved_console_loglevel == LOGLEVEL_DEFAULT)
> > > saved_console_loglevel = console_loglevel;
> > > + WARN_ONCE(minimum_console_loglevel < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", minimum_console_loglevel);
> > > console_loglevel = minimum_console_loglevel;
> > > break;
> > > /* Enable logging to console */
> > > case SYSLOG_ACTION_CONSOLE_ON:
> > > if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
> > > + WARN_ONCE(saved_console_loglevel < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", saved_console_loglevel);
> > > console_loglevel = saved_console_loglevel;
> > > saved_console_loglevel = LOGLEVEL_DEFAULT;
> > > }
> > > @@ -1548,6 +1550,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
> > > return -EINVAL;
> > > if (len < minimum_console_loglevel)
> > > len = minimum_console_loglevel;
> > > + WARN_ONCE(len < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", len);
> > > console_loglevel = len;
> > > /* Implicitly re-enable logging to console */
> > > saved_console_loglevel = LOGLEVEL_DEFAULT;
> > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > > index b3df3ab..2170421 100644
> > > --- a/kernel/sysctl.c
> > > +++ b/kernel/sysctl.c
> > > @@ -273,6 +273,27 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> > > int sysctl_legacy_va_layout;
> > > #endif
> > >
> > > +static int proc_dointvec_loglevel(struct ctl_table *table, int write,
> > > + void __user *buffer, size_t *lenp, loff_t *ppos)
> > > +{
> > > + if (write && buffer && *lenp) {
> > > + size_t len = *lenp;
> > > + char *kbuf, *p;
> > > +
> > > + if (len > PAGE_SIZE - 1)
> > > + len = PAGE_SIZE - 1;
> > > + p = kbuf = memdup_user_nul(buffer, len);
> > > + if (IS_ERR(kbuf))
> > > + return PTR_ERR(kbuf);
> > > + while (*p && (*p < '0' || *p > '9'))
> > > + p++;
> > > + len = *p ? strtoul(p, &p, 10) : CONSOLE_LOGLEVEL_DEFAULT;
> > > + WARN_ONCE(len < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", (int) len);
> > > + kfree(kbuf);
> > > + }
> > > + return proc_dointvec(table, write, buffer, lenp, ppos);
> > > +}
> > > +
> > > /* The default sysctl tables: */
> > >
> > > static struct ctl_table sysctl_base_table[] = {
> > > @@ -839,7 +860,7 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> > > .data = &console_loglevel,
> > > .maxlen = 4*sizeof(int),
> > > .mode = 0644,
> > > - .proc_handler = proc_dointvec,
> > > + .proc_handler = proc_dointvec_loglevel,
> > > },
> > > {
> > > .procname = "printk_ratelimit",
> > > --
> > > 1.8.3.1
> >
> >
> > Thanks.
> > Running this for 10 mins I only got a bunch of these, so at least none
> > of these WARNs does not trigger immediately. But I will continue
> > running it.
> >
> > WARNING in schedule_bh
> > kernel BUG at ./include/linux/mm.h:LINE!
> > WARNING in vkms_vblank_simulate
> > general protection fault in xfrmi_decode_session
> > INFO: task hung in corrupted
> > kernel BUG at drivers/android/binder_alloc.c:LINE!
> > BUG: unable to handle kernel NULL pointer dereference in
> > drm_atomic_helper_commit_modeset_disables
> > INFO: task hung in __rq_qos_throttle
> > WARNING in sk_stream_kill_queues
>
>
> Wait, but isn't SYSLOG_ACTION_CONSOLE_LEVEL what we are looking for?
> syzkaller knows about the syslog syscall:
> https://github.com/google/syzkaller/blob/13026d10f09f0e801c342e6c009ff580d49b894b/sys/linux/sys.txt#L322
> and even though it does not know SYSLOG_ACTION_CONSOLE_LEVEL const, it
> can guess that number.

I've restricted fuzzer from invoking SYSLOG_ACTION_CONSOLE_LEVEL.
Let's see if it helps.