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
