Messages in this thread | | | From | Ilias Apalodimas <> | Date | Thu, 15 Jul 2021 15:37:26 +0300 | Subject | Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling page_pool packets |
| |
On Thu, 15 Jul 2021 at 13:48, Yunsheng Lin <linyunsheng@huawei.com> wrote: > > On 2021/7/15 18:38, Ilias Apalodimas wrote: > > On Thu, 15 Jul 2021 at 13:00, Ilias Apalodimas > > <ilias.apalodimas@linaro.org> wrote: > >> > >> On Thu, 15 Jul 2021 at 07:01, Yunsheng Lin <linyunsheng@huawei.com> wrote: > >>> > >>> On 2021/7/9 14:29, Ilias Apalodimas wrote: > >>>> As Alexander points out, when we are trying to recycle a cloned/expanded > >>>> SKB we might trigger a race. The recycling code relies on the > >>>> pp_recycle bit to trigger, which we carry over to cloned SKBs. > >>>> If that cloned SKB gets expanded or if we get references to the frags, > >>>> call skbb_release_data() and overwrite skb->head, we are creating separate > >>>> instances accessing the same page frags. Since the skb_release_data() > >>>> will first try to recycle the frags, there's a potential race between > >>>> the original and cloned SKB, since both will have the pp_recycle bit set. > >>>> > >>>> Fix this by explicitly those SKBs not recyclable. > >>>> The atomic_sub_return effectively limits us to a single release case, > >>>> and when we are calling skb_release_data we are also releasing the > >>>> option to perform the recycling, or releasing the pages from the page pool. > >>>> > >>>> Fixes: 6a5bcd84e886 ("page_pool: Allow drivers to hint on SKB recycling") > >>>> Reported-by: Alexander Duyck <alexanderduyck@fb.com> > >>>> Suggested-by: Alexander Duyck <alexanderduyck@fb.com> > >>>> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org> > >>>> --- > >>>> Changes since v1: > >>>> - Set the recycle bit to 0 during skb_release_data instead of the > >>>> individual fucntions triggering the issue, in order to catch all > >>>> cases > >>>> net/core/skbuff.c | 4 +++- > >>>> 1 file changed, 3 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c > >>>> index 12aabcda6db2..f91f09a824be 100644 > >>>> --- a/net/core/skbuff.c > >>>> +++ b/net/core/skbuff.c > >>>> @@ -663,7 +663,7 @@ static void skb_release_data(struct sk_buff *skb) > >>>> if (skb->cloned && > >>>> atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1, > >>>> &shinfo->dataref)) > >>>> - return; > >>>> + goto exit; > >>> > >>> Is it possible this patch may break the head frag page for the original skb, > >>> supposing it's head frag page is from the page pool and below change clears > >>> the pp_recycle for original skb, causing a page leaking for the page pool? > >>> > >> > >> So this would leak eventually dma mapping if the skb is cloned (and > >> the dataref is now +1) and we are freeing the original skb first? > >> > > > > Apologies for the noise, my description was not complete. > > The case you are thinking is clone an SKB and then expand the original? > > Yes. > It seems we might need different pp_recycle bit for head frag and data frag.
We could just reset the pp_recycle flag on pskb_carve_inside_header, pskb_expand_header and pskb_carve_inside_nonlinear which were the three functions that might trigger the race to begin with. The point on adding it on skb_release_data was to have a catch all for all future cases ... Let me stare at itt a bit more in case I can come up with something better
Thanks /Ilias > > > > > thanks > > /Ilias > > > > > >>>> > >>>> skb_zcopy_clear(skb, true); > >>>> > >>>> @@ -674,6 +674,8 @@ static void skb_release_data(struct sk_buff *skb) > >>>> kfree_skb_list(shinfo->frag_list); > >>>> > >>>> skb_free_head(skb); > >>>> +exit: > >>>> + skb->pp_recycle = 0; > >>>> } > >>>> > >>>> /* > >>>> > > . > >
| |