[libvirt] [PATCH] qemuDomainBlockStatsFlags: Guard disk lookup with a domain job

[Peter Krempa]

On 03/08/13 13:20, Michal Privoznik wrote:
> When there are two concurrent threads, we may dereference a NULL
> pointer, even though it has been checked before:
>
> 1. Thread1: starts executing qemuDomainBlockStatsFlags() with nparams != 0.
>              It finds given disk and successfully pass check for disk->info.alias
>              not being NULL.
> 2. Thread2: starts executing qemuDomainDetachDeviceFlags() on the very same
>              disk as Thread1 is working on.
> 3. Thread1: gets to qemuDomainObjBeginJob() where it sets a job on a
>              domain.
> 4. Thread2: also tries to set a job. However, we are not guaranteed which
>              thread wins. So assume it's Thread2 who can continue.
> 5. Thread2: does the actual detach and frees disk->info.alias
> 6. Thread2: quits the job
> 7. Thread1: now successfully acquires the job, and accesses a NULL pointer.
> ---

Wow! Yes that might happen,

>   src/qemu/qemu_driver.c | 18 ++++++------------
>   1 file changed, 6 insertions(+), 12 deletions(-)
>

ACK.

Peter