linux-kernel - Re: [PATCH kernel] srcu: Fix static initialization

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Date:   Wed, 9 Sep 2020 19:24:11 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     rcu@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel] srcu: Fix static initialization



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@...abs.ru>
> ---
>  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?


> +
>  	/*
>  	 * 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