Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop

[Cédric Le Goater]

On 03/11/2016 07:48 PM, Greg Kurz wrote:
> Since commit "60253ed1e6ec rng: add request queue support to rng-random",
> the use of a spapr_rng device may hang vCPU threads.
> 
> The following path is taken without holding the lock to the main loop mutex:
> 
> h_random()
>   rng_backend_request_entropy()
>     rng_random_request_entropy()
>       qemu_set_fd_handler()
> 
> The consequence is that entropy_available() may be called before the vCPU
> thread could even queue the request: depending on the scheduling, it may
> happen that entropy_available() does not call random_recv()->qemu_sem_post().
> The vCPU thread will then sleep forever in h_random()->qemu_sem_wait().
> 
> This could not happen before 60253ed1e6ec because entropy_available() used
> to call random_recv() unconditionally.
> 
> This patch ensures the lock is held to avoid the race.
> 
> Signed-off-by: Greg Kurz <address@hidden>

Reviewed-by: Cédric Le Goater <address@hidden>

Thanks for finding this race,

C.
> ---
> 
> Thomas,
> 
> This is the problem mentioned by Cedric in:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02526.html
> 
> Cheers.
> 
> --
> Greg
> 
>  hw/ppc/spapr_rng.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr_rng.c b/hw/ppc/spapr_rng.c
> index a39d472b66fd..02d6be49f58e 100644
> --- a/hw/ppc/spapr_rng.c
> +++ b/hw/ppc/spapr_rng.c
> @@ -77,13 +77,13 @@ static target_ulong h_random(PowerPCCPU *cpu, 
> sPAPRMachineState *spapr,
>      hrdata.val.v64 = 0;
>      hrdata.received = 0;
> 
> -    qemu_mutex_unlock_iothread();
>      while (hrdata.received < 8) {
>          rng_backend_request_entropy(rngstate->backend, 8 - hrdata.received,
>                                      random_recv, &hrdata);
> +        qemu_mutex_unlock_iothread();
>          qemu_sem_wait(&hrdata.sem);
> +        qemu_mutex_lock_iothread();
>      }
> -    qemu_mutex_lock_iothread();
> 
>      qemu_sem_destroy(&hrdata.sem);
>      args[0] = hrdata.val.v64;
>