Messages in this thread | | | Subject | Re: [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize incorrectly | From | Eric Dumazet <> | Date | Mon, 23 Aug 2021 15:23:29 -0700 |
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On 8/23/21 2:51 PM, Eric Dumazet wrote: > > > On 8/23/21 2:45 PM, Eric Dumazet wrote: >> >> >> On 8/23/21 10:25 AM, Christoph Paasch wrote: >>> Hello, >>> >>> On Mon, Aug 23, 2021 at 12:56 AM Vasily Averin <vvs@virtuozzo.com> wrote: >>>> >>>> Christoph Paasch reports [1] about incorrect skb->truesize >>>> after skb_expand_head() call in ip6_xmit. >>>> This happen because skb_set_owner_w() for newly clone skb is called >>>> too early, before pskb_expand_head() where truesize is adjusted for >>>> (!skb-sk) case. >>>> >>>> [1] https://lkml.org/lkml/2021/8/20/1082 >>>> >>>> Reported-by: Christoph Paasch <christoph.paasch@gmail.com> >>>> Signed-off-by: Vasily Averin <vvs@virtuozzo.com> >>>> --- >>>> net/core/skbuff.c | 24 +++++++++++++----------- >>>> 1 file changed, 13 insertions(+), 11 deletions(-) >>>> >>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c >>>> index f931176..508d5c4 100644 >>>> --- a/net/core/skbuff.c >>>> +++ b/net/core/skbuff.c >>>> @@ -1803,6 +1803,8 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom) >>>> >>>> struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom) >>>> { >>>> + struct sk_buff *oskb = skb; >>>> + struct sk_buff *nskb = NULL; >>>> int delta = headroom - skb_headroom(skb); >>>> >>>> if (WARN_ONCE(delta <= 0, >>>> @@ -1811,21 +1813,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom) >>>> >>>> /* pskb_expand_head() might crash, if skb is shared */ >>>> if (skb_shared(skb)) { >>>> - struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC); >>>> - >>>> - if (likely(nskb)) { >>>> - if (skb->sk) >>>> - skb_set_owner_w(nskb, skb->sk); >>>> - consume_skb(skb); >>>> - } else { >>>> - kfree_skb(skb); >>>> - } >>>> + nskb = skb_clone(skb, GFP_ATOMIC); >>>> skb = nskb; >>>> } >>>> if (skb && >>>> - pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) { >>>> - kfree_skb(skb); >>>> + pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) >>>> skb = NULL; >>>> + >>>> + if (!skb) { >>>> + kfree_skb(oskb); >>>> + if (nskb) >>>> + kfree_skb(nskb); >>>> + } else if (nskb) { >>>> + if (oskb->sk) >>>> + skb_set_owner_w(nskb, oskb->sk); >>>> + consume_skb(oskb); >>> >>> sorry, this does not fix the problem. The syzkaller repro still >>> triggers the WARN. >>> >>> When it happens, the skb in ip6_xmit() is not shared as it comes from >>> __tcp_transmit_skb, where it is skb_clone()'d. >>> >>> >> >> Old code (in skb_realloc_headroom()) >> was first calling skb2 = skb_clone(skb, GFP_ATOMIC); >> >> At this point, skb2->sk was NULL >> So pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0, ...) was able to tweak skb2->truesize >> >> I would try : >> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c >> index f9311762cc475bd38d87c33e988d7c983b902e56..326749a8938637b044a616cc33b6a19ed191ac41 100644 >> --- a/net/core/skbuff.c >> +++ b/net/core/skbuff.c >> @@ -1804,6 +1804,7 @@ EXPORT_SYMBOL(skb_realloc_headroom); >> struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom) >> { >> int delta = headroom - skb_headroom(skb); >> + struct sk_buff *oskb = NULL; >> >> if (WARN_ONCE(delta <= 0, >> "%s is expecting an increase in the headroom", __func__)) >> @@ -1813,19 +1814,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom) >> if (skb_shared(skb)) { >> struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC); >> >> - if (likely(nskb)) { >> - if (skb->sk) >> - skb_set_owner_w(nskb, skb->sk); >> - consume_skb(skb); >> - } else { >> + if (unlikely(!nskb)) { >> kfree_skb(skb); >> + return NULL; >> } >> + oskb = skb; >> skb = nskb; >> } >> - if (skb && >> - pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) { >> + if (pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) { >> kfree_skb(skb); >> - skb = NULL; >> + kfree_skb(oskb); >> + return NULL; >> + } >> + if (oskb) { >> + skb_set_owner_w(skb, oskb->sk); >> + consume_skb(oskb); >> } >> return skb; >> } > > > Oh well, probably not going to work. > > We have to find a way to properly increase skb->truesize, even if skb_clone() is _not_ called. >
I also note that current use of skb_set_owner_w(), forcing skb->destructor to sock_wfree() is probably breaking TCP Small queues, since original skb->destructor would be tcp_wfree() or __sock_wfree()
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