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SubjectRe: [Linux 5.2.x] /sys/kernel/debug/tracing/events/power/cpu_idle/id: BUG: kernel NULL pointer dereference, address: 0000000000000000
hi,

On Sun, Aug 11, 2019 at 08:22:35AM +0200, Thomas Gleixner wrote:
> On Sat, 10 Aug 2019, Paul Menzel wrote:
>
> Cc+ Steven
>
> > [+ INTEL IDLE DRIVER]
> >
> > Dear Linux folks,
> >
> >
> > On 10.08.19 20:28, Paul Menzel wrote:
> >
> > > On 10.08.19 19:31, Thomas Gleixner wrote:
> > >
> > > > On Sat, 10 Aug 2019, Paul Menzel wrote:
> > > > >
> > > > > I have no idea, who to report this to, so I please refer me to the
> > > > > correct
> > > > > list.
> > > >
> > > > I have no idea yet either :)
> > > >
> > > > > With Linux 5.2.7 from Debian Sid/unstable and PowerTOP 2.10, executing
> > > > >
> > > > >      sudo powertop --auto-tune
> > > > >
> > > > > causes a NULL pointer dereference, and the graphical session crashes due
> > > > > to an
> > > > > effect on the i915 driver. It worked in the past with the 4.19 series
> > > > > from
> > > > > Debian.
> > > > >
> > > > > Here is the trace, and please find all Linux kernel logs attached.
> > > > >
> > > > > > [ 2027.170589] BUG: kernel NULL pointer dereference, address:
> > > > > > 0000000000000000
> > > > > > [ 2027.170600] #PF: supervisor instruction fetch in kernel mode
> > > > > > [ 2027.170604] #PF: error_code(0x0010) - not-present page
> > > > > > [ 2027.170609] PGD 0 P4D 0 [ 2027.170619] Oops: 0010 [#1] SMP PTI
> > > > ...
> > > > > > [ 2027.170730]  do_dentry_open+0x13a/0x370
> > > >
> > > > If you have compiled with debug info, please decode the line:
> > > >
> > > >    linux/scripts/faddr2line vmlinux do_dentry_open+0x13a/0x370
> > > >
> > > > That gives us the fops pointer which is NULL.
> > >
> > > Hah, luckily it’s reproducible.
> > >
> > > ```
> > > $ scripts/faddr2line /usr/lib/debug/boot/vmlinux-5.2.0-2-amd64
> > > do_dentry_open+0x13a/0x370
> > > do_dentry_open+0x13a/0x370:
> > > do_dentry_open at fs/open.c:799
> > > ```
> > >
> > > > > > [ 2027.170745]  path_openat+0x2c6/0x1480
> > > > > > [ 2027.170757]  ? terminate_walk+0xe6/0x100
> > > > > > [ 2027.170767]  ? path_lookupat.isra.48+0xa3/0x220
> > > > > > [ 2027.170779]  ? reuse_swap_page+0x105/0x320
> > > > > > [ 2027.170791]  do_filp_open+0x93/0x100
> > > > > > [ 2027.170804]  ? __check_object_size+0x15d/0x189
> > > > > > [ 2027.170816]  do_sys_open+0x184/0x220
> > > > > > [ 2027.170828]  do_syscall_64+0x53/0x130
> > > > > > [ 2027.170837]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > >
> > > > That's an open crashing. We just don't know which file. Is the machine
> > > > completely hosed after that or is it just the graphics stuff dying?
> > >
> > > No, the graphical login manager showed up, and I could log back in, and
> > > continue using hte machine.
> > >
> > > > If it's not completely dead then instead of running it from your graphical
> > > > desktop you could switch to a VGA terminal Alt+Ctrl+F1 (or whatever
> > > > function key your distro maps to) after boot and run powertop with strace
> > > > from there:
> > > >
> > > >    strace -f -o xxx.log powertop
> > > >
> > > > With a bit of luck xxx.log should contain the information about the file
> > > > it
> > > > tries to open.
> > >
> > > ```
> > > 2157  access("/sys/class/drm/card0/power/rc6_residency_ms", R_OK) = 0
> > > 2157  openat(AT_FDCWD, "/sys/kernel/debug/tracing/events/power/cpu_idle/id",
> > > O_RDONLY) = ?
> > > 2157  +++ killed by SIGKILL +++
> > > ```
> > >
> > > > Alternatively if you have a serial console you can enable the
> > > > sys_enter_open* tracepoints:
> > > >
> > > > # echo 1 >/sys/kernel/debug/tracing/events/syscalls/sys_enter_open
> > > > # echo 1 >/sys/kernel/debug/tracing/events/syscalls/sys_enter_openat
> > > >
> > > > Either add 'ftrace_dump_on_oops' to the kernel command line or enable it
> > > > from the shell:
> > > >
> > > > # echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
> > > >
> > > > Then run powertop. After the crash it will take some time to spill out the
> > > > trace buffer over serial, but it will pinpoint the offending file.
> > >
> > > I do not have serial console on this device.
> >
> > For the record. It is also reproducible with Linux 5.2.6, and trying to print
> > the file contents with cat already fails.
> >
> > ```
> > $ sudo ls -l /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > -r--r--r-- 1 root root 0 Aug 10 23:05
> > /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > $ sudo cat /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > Killed
> > ```

This seems to be related to https://bugs.debian.org/934304 (in
particular https://bugs.debian.org/934304#29). The mentioned patch
features/all/lockdown/0031-tracefs-Restrict-tracefs-when-the-kernel-is-locked-d.patch
is a backport of https://patchwork.kernel.org/patch/11069661/ with
only change that it is converted back to the non-LSM lockdown API.

Regards,
Salvatore

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