Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jul 26, 2017 at 01:05:23PM +0100, Matt Fleming wrote:
> On Tue, 25 Jul, at 11:04:39AM, Greg KH wrote:
> > On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> > > From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> > > 
> > > commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> > > 
> > > Commit:
> > > 
> > >   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > 
> > > .. moved sched_online_group() from css_online() to css_alloc().
> > > It exposes half-baked task group into global lists before initializing
> > > generic cgroup stuff.
> > > 
> > > LTP testcase (third in cgroup_regression_test) written for testing
> > > similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> > > 
> > >   BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> > >   IP: kernfs_path_from_node_locked+0x260/0x320
> > >   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
> > >   Call Trace:
> > >   ? kernfs_path_from_node+0x4f/0x60
> > >   kernfs_path_from_node+0x3e/0x60
> > >   print_rt_rq+0x44/0x2b0
> > >   print_rt_stats+0x7a/0xd0
> > >   print_cpu+0x2fc/0xe80
> > >   ? __might_sleep+0x4a/0x80
> > >   sched_debug_show+0x17/0x30
> > >   seq_read+0xf2/0x3b0
> > >   proc_reg_read+0x42/0x70
> > >   __vfs_read+0x28/0x130
> > >   ? security_file_permission+0x9b/0xc0
> > >   ? rw_verify_area+0x4e/0xb0
> > >   vfs_read+0xa5/0x170
> > >   SyS_read+0x46/0xa0
> > >   entry_SYSCALL_64_fastpath+0x1e/0xad
> > > 
> > > Here the task group is already linked into the global RCU-protected 'task_groups'
> > > list, but the css->cgroup pointer is still NULL.
> > > 
> > > This patch reverts this chunk and moves online back to css_online().
> > > 
> > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Tejun Heo <tj@kernel.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > Link: http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> > > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > > Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
> > > ---
> > >  kernel/sched/core.c | 14 ++++++++++++--
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > What about 4.9-stable, this should go there too, right?
> 
> Yes, good catch. Would you like me to send a separate patch?

If it needs a backport and a simple cherry-pick does not work, yes
please.

thanks,

greg k-h