RE: [dpdk-dev] [PATCH v2] ring: fix use after free in ring release

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> >>
> >>
> >>
> >>> After the memzone is freed, it is not removed from the 'rte_ring_tailq'.
> >>> If rte_ring_lookup is called at this time, it will cause a
> >>> use-after-free problem. This change prevents that from happening.
> >>>
> >>> Fixes: 4e32101f9b01 ("ring: support freeing")
> >>> Cc: stable@dpdk.org
> >>>
> >>> Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> >>> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> >>> ---
> >>> v2: update code suggested by Honnappa Nagarahalli
> >>> ---
> >>>   lib/ring/rte_ring.c | 8 +++-----
> >>>   1 file changed, 3 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/lib/ring/rte_ring.c b/lib/ring/rte_ring.c index
> >>> 8ed455043d..2755323b8a 100644
> >>> --- a/lib/ring/rte_ring.c
> >>> +++ b/lib/ring/rte_ring.c
> >>> @@ -333,11 +333,6 @@ rte_ring_free(struct rte_ring *r)
> >>>   		return;
> >>>   	}
> >>>
> >>> -	if (rte_memzone_free(r->memzone) != 0) {
> >>> -		RTE_LOG(ERR, RING, "Cannot free memory\n");
> >>> -		return;
> >>> -	}
> >>> -
> >>>   	ring_list = RTE_TAILQ_CAST(rte_ring_tailq.head, rte_ring_list);
> >>>   	rte_mcfg_tailq_write_lock();
> >>>
> >>> @@ -354,6 +349,9 @@ rte_ring_free(struct rte_ring *r)
> >>>
> >>>   	TAILQ_REMOVE(ring_list, te, next);
> >>>
> >>> +	if (rte_memzone_free(r->memzone) != 0)
> >>> +		RTE_LOG(ERR, RING, "Cannot free memory\n");
> >>> +
> >>
> >> I nit: I think it is a bit better to first release the lock and then
> >> free the memzone.
> > I think both of our suggestions are contradictory. Any reason why you want
> to free outside the locked region?
> 
> 
> Don't know what you mean by 'both suggestions' here.
I wrote 'both of our suggestions'. Essentially, in v1, freeing the memzone was outside of the lock. I suggested to move it inside and you are suggesting to move it inside.

> I think I gave only one - move memzone_free() after tailq_write_unlock().
> To be more precise:
> 1) rte_mcfg_tailq_write_lock();
> ...
> 2) TAILQ_REMOVE(...);
> 3) rte_mcfg_tailq_write_unlock();
> 4) rte_memzone_free(r->memzone);
> 
> As I remember, memzones are protected by their own lock (mlock), so we
> don't need to hold qlock to free a memzone, after ring was already removed
> from the ring_list.
> 
> >
> > I thought, since it belongs to the ring being freed, it makes sense to free it
> while holding the lock to avoid any race conditions (though, I have not
> checked what those are).
> 
> 
> As I understand, it is ok with current design to grab mlock while holding qlock.
> So, there is nothing wrong with current patch, I just think that in that case it is
> excessive, and can be safely avoided.
> 
> >
> >> Apart from that, LGTM.
> >> Acked-by: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>
> >>
> >>>   	rte_mcfg_tailq_write_unlock();
> >>>
> >>>   	rte_free(te);
> >>> --
> >>> 2.33.0
> >>
> >