Re: [tip: irq/urgent] softirq: Fix suspicious RCU usage in __do_softirq()

[Michael S. Tsirkin]

On Mon, Apr 29, 2024 at 03:08:02AM -0000, tip-bot2 for Zqiang wrote:
> The following commit has been merged into the irq/urgent branch of tip:
> 
> Commit-ID:     1dd1eff161bd55968d3d46bc36def62d71fb4785
> Gitweb:        https://git.kernel.org/tip/1dd1eff161bd55968d3d46bc36def62d71fb4785
> Author:        Zqiang <qiang.zhang1211@xxxxxxxxx>
> AuthorDate:    Sat, 27 Apr 2024 18:28:08 +08:00
> Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CommitterDate: Mon, 29 Apr 2024 05:03:51 +02:00
> 
> softirq: Fix suspicious RCU usage in __do_softirq()
> 
> Currently, the condition "__this_cpu_read(ksoftirqd) == current" is used to
> invoke rcu_softirq_qs() in ksoftirqd tasks context for non-RT kernels.
> 
> This works correctly as long as the context is actually task context but
> this condition is wrong when:
> 
>      - the current task is ksoftirqd
>      - the task is interrupted in a RCU read side critical section
>      - __do_softirq() is invoked on return from interrupt
> 
> Syzkaller triggered the following scenario:
> 
>   -> finish_task_switch()
>     -> put_task_struct_rcu_user()
>       -> call_rcu(&task->rcu, delayed_put_task_struct)
>         -> __kasan_record_aux_stack()
>           -> pfn_valid()
>             -> rcu_read_lock_sched()
>               <interrupt>
>                 __irq_exit_rcu()
>                 -> __do_softirq)()
>                    -> if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
>                      __this_cpu_read(ksoftirqd) == current)
>                      -> rcu_softirq_qs()
>                        -> RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map))
> 
> The rcu quiescent state is reported in the rcu-read critical section, so
> the lockdep warning is triggered.
> 
> Fix this by splitting out the inner working of __do_softirq() into a helper
> function which takes an argument to distinguish between ksoftirqd task
> context and interrupted context and invoke it from the relevant call sites
> with the proper context information and use that for the conditional
> invocation of rcu_softirq_qs().
> 
> Reported-by: syzbot+dce04ed6d1438ad69656@xxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20240427102808.29356-1-qiang.zhang1211@xxxxxxxxx
> Link: https://lore.kernel.org/lkml/8f281a10-b85a-4586-9586-5bbc12dc784f@paulmck-laptop/T/#mea8aba4abfcb97bbf499d169ce7f30c4cff1b0e3

I can add that this also fixes a UAF reported by syzbot
(partially, another part of UAF is an unrelated bug):

Reported-by: syzbot+98edc2df894917b3431f@xxxxxxxxxxxxxxxxxxxxxxxxx



> ---
>  kernel/softirq.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index b315b21..0258201 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -508,7 +508,7 @@ static inline bool lockdep_softirq_start(void) { return false; }
>  static inline void lockdep_softirq_end(bool in_hardirq) { }
>  #endif
>  
> -asmlinkage __visible void __softirq_entry __do_softirq(void)
> +static void handle_softirqs(bool ksirqd)
>  {
>  	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
>  	unsigned long old_flags = current->flags;
> @@ -563,8 +563,7 @@ restart:
>  		pending >>= softirq_bit;
>  	}
>  
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> -	    __this_cpu_read(ksoftirqd) == current)
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && ksirqd)
>  		rcu_softirq_qs();
>  
>  	local_irq_disable();
> @@ -584,6 +583,11 @@ restart:
>  	current_restore_flags(old_flags, PF_MEMALLOC);
>  }
>  
> +asmlinkage __visible void __softirq_entry __do_softirq(void)
> +{
> +	handle_softirqs(false);
> +}
> +
>  /**
>   * irq_enter_rcu - Enter an interrupt context with RCU watching
>   */
> @@ -921,7 +925,7 @@ static void run_ksoftirqd(unsigned int cpu)
>  		 * We can safely run softirq on inline stack, as we are not deep
>  		 * in the task stack here.
>  		 */
> -		__do_softirq();
> +		handle_softirqs(true);
>  		ksoftirqd_run_end();
>  		cond_resched();
>  		return;