* [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
@ 2016-03-11 18:48 Greg Kurz
2016-03-14 10:03 ` Cédric Le Goater
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Greg Kurz @ 2016-03-11 18:48 UTC (permalink / raw)
To: David Gibson; +Cc: Thomas Huth, Cedric Le Goater, qemu-ppc, qemu-devel
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;
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
2016-03-11 18:48 [Qemu-devel] [PATCH] spapr_rng: fix race with main loop Greg Kurz
@ 2016-03-14 10:03 ` Cédric Le Goater
2016-03-14 10:51 ` Thomas Huth
2016-03-15 0:30 ` David Gibson
2 siblings, 0 replies; 4+ messages in thread
From: Cédric Le Goater @ 2016-03-14 10:03 UTC (permalink / raw)
To: Greg Kurz, David Gibson; +Cc: Thomas Huth, qemu-ppc, qemu-devel
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;
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
2016-03-11 18:48 [Qemu-devel] [PATCH] spapr_rng: fix race with main loop Greg Kurz
2016-03-14 10:03 ` Cédric Le Goater
@ 2016-03-14 10:51 ` Thomas Huth
2016-03-15 0:30 ` David Gibson
2 siblings, 0 replies; 4+ messages in thread
From: Thomas Huth @ 2016-03-14 10:51 UTC (permalink / raw)
To: Greg Kurz, David Gibson
Cc: Cedric Le Goater, qemu-ppc, qemu-devel, Ladi Prosek
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>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
2016-03-11 18:48 [Qemu-devel] [PATCH] spapr_rng: fix race with main loop Greg Kurz
2016-03-14 10:03 ` Cédric Le Goater
2016-03-14 10:51 ` Thomas Huth
@ 2016-03-15 0:30 ` David Gibson
2 siblings, 0 replies; 4+ messages in thread
From: David Gibson @ 2016-03-15 0:30 UTC (permalink / raw)
To: Greg Kurz; +Cc: Thomas Huth, Cedric Le Goater, qemu-ppc, qemu-devel
[-- 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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^ permalink raw reply [flat|nested] 4+ messages in thread
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2016-03-11 18:48 [Qemu-devel] [PATCH] spapr_rng: fix race with main loop Greg Kurz
2016-03-14 10:03 ` Cédric Le Goater
2016-03-14 10:51 ` Thomas Huth
2016-03-15 0:30 ` David Gibson
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