[PATCH] nwfilter_driver: Make reload fail if racing with stateCleanup()

[Daniel P. Berrangé]

On Fri, Apr 22, 2022 at 04:45:37PM +0200, Michal Privoznik wrote:
> When one thread is trying to reload NWFilter driver (by running
> nwfilterStateReload()) but there's another thread that's
> concurrently running nwfilterStateCleanup() a crash may occur.
> This is despite nwfilterStateReload() checking for driver !=
> NULL, because is done so without @driverMutex held. A typical
> TOCTOU. Fortunately, the mutex is always initialized, so the
> mutex can be acquired at all times and @driver can be checked
> with the lock held.
> 
> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2075837
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/nwfilter/nwfilter_driver.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)

I'm concerned that we might have latent bugs in other
drivers too, and/or be at risk of introducing them
later.

I've always thought of StateReload as been non-overlapping
with StateCleanup, though I realize now that's not actually
the case in practice.

I wonder if it would better if we made remote_daemon.c
avoiding calling StateCleanup, until any StateReload
has finished, to eliminate the entire class of problem
across all the drivers.

> 
> diff --git a/src/nwfilter/nwfilter_driver.c b/src/nwfilter/nwfilter_driver.c
> index b66ba22737..d028efafbe 100644
> --- a/src/nwfilter/nwfilter_driver.c
> +++ b/src/nwfilter/nwfilter_driver.c
> @@ -309,6 +309,8 @@ nwfilterStateInitialize(bool privileged,
>  static int
>  nwfilterStateReload(void)
>  {
> +    VIR_LOCK_GUARD lock = virLockGuardLock(&driverMutex);
> +
>      if (!driver)
>          return -1;
>  
> @@ -319,15 +321,13 @@ nwfilterStateReload(void)
>      /* shut down all threads -- they will be restarted if necessary */
>      virNWFilterLearnThreadsTerminate(true);
>  
> -    VIR_WITH_MUTEX_LOCK_GUARD(&driverMutex) {
> -        VIR_WITH_MUTEX_LOCK_GUARD(&driver->updateLock) {
> -            virNWFilterObjListLoadAllConfigs(driver->nwfilters, driver->configDir);
> -        }
> -
> -
> -        virNWFilterBuildAll(driver, false);
> +    VIR_WITH_MUTEX_LOCK_GUARD(&driver->updateLock) {
> +        virNWFilterObjListLoadAllConfigs(driver->nwfilters, driver->configDir);
>      }
>  
> +
> +    virNWFilterBuildAll(driver, false);
> +
>      return 0;
>  }
>  
> -- 
> 2.35.1
> 

With regards,
Daniel
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