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

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

[Greg Kurz]

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 <gkurz@linux.vnet.ibm.com>
---

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;

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 <gkurz@linux.vnet.ibm.com>

Reviewed-by: Cédric Le Goater <clg@fr.ibm.com>

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;
> 

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

[Thomas Huth]

On 11.03.2016 19:48, 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 <gkurz@linux.vnet.ibm.com>
> ---
> 
> 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;

Reviewed-by: Thomas Huth <thuth@redhat.com>

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

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1349 bytes --]

On Fri, Mar 11, 2016 at 07:48:47PM +0100, 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 <gkurz@linux.vnet.ibm.com>

Applied to ppc-for-2.6

> ---
> 
> Thomas,
> 
> This is the problem mentioned by Cedric in:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02526.html
> 
> Cheers.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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