Re: [PATCH kernel] srcu: Fix static initialization

[Paul E. McKenney]

On Wed, Sep 09, 2020 at 07:24:11PM +1000, Alexey Kardashevskiy wrote:
> 
> 
> On 09/09/2020 00:43, Alexey Kardashevskiy wrote:
> > init_srcu_struct_nodes() is called with is_static==true only internally
> > and when this happens, the srcu->sda is not initialized in
> > init_srcu_struct_fields() and we crash on dereferencing @sdp.
> > 
> > This fixes the crash by moving "if (is_static)" out of the loop which
> > only does useful work for is_static=false case anyway.
> > 
> > Found by syzkaller.
> > 
> > Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> > ---
> >  kernel/rcu/srcutree.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index c100acf332ed..49b54a50bde8 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -135,6 +135,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
> >  				   levelspread[level - 1];
> >  	}
> >  
> > +	if (is_static)
> > +		return;
> 
> Actually, this is needed here too:
> 
>  if (!ssp->sda)
>          return;
> 
> as
> ssp->sda = alloc_percpu(struct srcu_data)
> 
> can fail if the process is killed too soon - it is quite easy to get
> this situation with syzkaller (syscalls fuzzer)
> 
> Makes sense?

Just to make sure that I understand, these failures occur when the task
running init_srcu_struct_nodes() is killed, correct?

Or has someone managed to invoke (say) synchronize_srcu() on a
dynamically allocated srcu_struct before invoking init_srcu_struct() on
that srcu_struct?  This would be an SRCU usage bug.  If you dynamically
allocate your srcu_struct, you are absolutely required to invoke
init_srcu_struct() on it before doing anything else with it.

Or am I missing something here?

(The rcutorture test suite does test both static and dynamic allocation
of the srcu_struct, so I am expecting something a bit subtle here.)

							Thanx, Paul

> > +
> >  	/*
> >  	 * Initialize the per-CPU srcu_data array, which feeds into the
> >  	 * leaves of the srcu_node tree.
> > @@ -161,8 +164,6 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
> >  		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
> >  		sdp->ssp = ssp;
> >  		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
> > -		if (is_static)
> > -			continue;
> >  
> >  		/* Dynamically allocated, better be no srcu_read_locks()! */
> >  		for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
> > 
> 
> -- 
> Alexey