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Date: Wed, 21 Mar 2018 07:59:15 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Eric Dumazet <edumazet@...gle.com>, ast@...com,
Daniel Borkmann <daniel@...earbox.net>, diptanu@...com,
Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net-next v3 1/2] net: permit skb_segment on head_frag
frag_list skb
On Tue, Mar 20, 2018 at 10:02 PM, Yonghong Song <yhs@...com> wrote:
>
>
> On 3/20/18 4:50 PM, Alexander Duyck wrote:
>>
>> On Tue, Mar 20, 2018 at 4:21 PM, Yonghong Song <yhs@...com> wrote:
>>>
>>> One of our in-house projects, bpf-based NAT, hits a kernel BUG_ON at
>>> function skb_segment(), line 3667. The bpf program attaches to
>>> clsact ingress, calls bpf_skb_change_proto to change protocol
>>> from ipv4 to ipv6 or from ipv6 to ipv4, and then calls bpf_redirect
>>> to send the changed packet out.
>>>
>>> 3472 struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>> 3473 netdev_features_t features)
>>> 3474 {
>>> 3475 struct sk_buff *segs = NULL;
>>> 3476 struct sk_buff *tail = NULL;
>>> ...
>>> 3665 while (pos < offset + len) {
>>> 3666 if (i >= nfrags) {
>>> 3667 BUG_ON(skb_headlen(list_skb));
>>> 3668
>>> 3669 i = 0;
>>> 3670 nfrags =
>>> skb_shinfo(list_skb)->nr_frags;
>>> 3671 frag = skb_shinfo(list_skb)->frags;
>>> 3672 frag_skb = list_skb;
>>> ...
>>>
>>> call stack:
>>> ...
>>> #1 [ffff883ffef03558] __crash_kexec at ffffffff8110c525
>>> #2 [ffff883ffef03620] crash_kexec at ffffffff8110d5cc
>>> #3 [ffff883ffef03640] oops_end at ffffffff8101d7e7
>>> #4 [ffff883ffef03668] die at ffffffff8101deb2
>>> #5 [ffff883ffef03698] do_trap at ffffffff8101a700
>>> #6 [ffff883ffef036e8] do_error_trap at ffffffff8101abfe
>>> #7 [ffff883ffef037a0] do_invalid_op at ffffffff8101acd0
>>> #8 [ffff883ffef037b0] invalid_op at ffffffff81a00bab
>>> [exception RIP: skb_segment+3044]
>>> RIP: ffffffff817e4dd4 RSP: ffff883ffef03860 RFLAGS: 00010216
>>> RAX: 0000000000002bf6 RBX: ffff883feb7aaa00 RCX: 0000000000000011
>>> RDX: ffff883fb87910c0 RSI: 0000000000000011 RDI: ffff883feb7ab500
>>> RBP: ffff883ffef03928 R8: 0000000000002ce2 R9: 00000000000027da
>>> R10: 000001ea00000000 R11: 0000000000002d82 R12: ffff883f90a1ee80
>>> R13: ffff883fb8791120 R14: ffff883feb7abc00 R15: 0000000000002ce2
>>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
>>> #9 [ffff883ffef03930] tcp_gso_segment at ffffffff818713e7
>>> --- <IRQ stack> ---
>>> ...
>>>
>>> The triggering input skb has the following properties:
>>> list_skb = skb->frag_list;
>>> skb->nfrags != NULL && skb_headlen(list_skb) != 0
>>> and skb_segment() is not able to handle a frag_list skb
>>> if its headlen (list_skb->len - list_skb->data_len) is not 0.
>>>
>>> This patch addressed the issue by handling skb_headlen(list_skb) != 0
>>> case properly if list_skb->head_frag is true, which is expected in
>>> most cases. The head frag is processed before list_skb->frags
>>> are processed.
>>>
>>> Reported-by: Diptanu Gon Choudhury <diptanu@...com>
>>> Signed-off-by: Yonghong Song <yhs@...com>
>>> ---
>>> net/core/skbuff.c | 51
>>> +++++++++++++++++++++++++++++++++++++--------------
>>> 1 file changed, 37 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index 715c134..59bbc06 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -3475,7 +3475,7 @@ struct sk_buff *skb_segment(struct sk_buff
>>> *head_skb,
>>> struct sk_buff *segs = NULL;
>>> struct sk_buff *tail = NULL;
>>> struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list;
>>> - skb_frag_t *frag = skb_shinfo(head_skb)->frags;
>>> + skb_frag_t *frag = skb_shinfo(head_skb)->frags, *head_frag =
>>> NULL;
>>
>>
>> I think you misunderstood me. I wasn't saying you allocate head_frag.
>> I was saying you could move the declaration down.
>
>
> Sorry for my misunderstanding. I did understand your intention of moving
> the declaration down in order to save stack space. I thought that we cannot
> really move declaration down (although it works in C, but semantically it is
> not quite right, more later), so I moved on to
> use runtime allocation. But indeed skb_frag_t is not big (16 bytes), it
> could live on the stack.
>
>>
>>> unsigned int mss = skb_shinfo(head_skb)->gso_size;
>>> unsigned int doffset = head_skb->data -
>>> skb_mac_header(head_skb);
>>> struct sk_buff *frag_skb = head_skb;
>>> @@ -3664,19 +3664,39 @@ struct sk_buff *skb_segment(struct sk_buff
>>> *head_skb,
>>>
>>> while (pos < offset + len) {
>>
>>
>> So right here in the loop you could add a "skb_frag_t head_frag;" just
>> so we declare it here and save ourselves the stack space.
>
>
> I actually tried to move "skb_frag_t head_frag". The stack size remains the
> same, 0xc0. This is related to how C compiler allocates stack space.
> The declaration place won't decide the stack size as long as the declaration
> dictates the usage. The stack size is really determined by liveness
> analysis.
>
> Further, we have code like:
> do {
> ....
> while (pos < offset + len) {
> if (i >= nfrags) {
> ...
> head_frag = ...
> }
> ... = head_frag; // head_frag access guaranteed after
> // above definition, but it may not
> // be in the same outer do-while loop.
> }
> ...
> } while (((offset += len) < head_skb->len);
>
> So the use of head_frag maybe in different outer loop iterations.
> So I feel the definition of head_frag should be outside the
> outer do-while loop, which is the main function scope. I will add some
> comments here.
So the point I had is that head_frag doesn't need to live that long.
All you are doing is arranging the data so that you can essentially
just dump it into *nskb_frag.
One alternative you could look at would be to rearrange the code so
that the MAX_SKB_FRAGS check occurs first, then perform the check for
(i >= nfrags). Doing it that way we end up bypassing the need for
head_frag entirely as you could just populate *nskb_frag directly. You
could probably just add an inline structure that would convert the
head frag into a skb_frag_t structure and return that. Something along
the lines of:
*nskb_frag = (i < 0) ? skb_head_frag_to_page_desc(frag_skb) : *frag;
Doing it that way you could pull out the bits below that were
populating head frag and just have it all handled as an inline
function. An added advantage is that it should make the line wrapping
easier to deal with. :-)
>>
>>> if (i >= nfrags) {
>>> - BUG_ON(skb_headlen(list_skb));
>>> -
>>> i = 0;
>>> + if (skb_headlen(list_skb)) {
>>> + struct page *page;
>>> +
>>> + BUG_ON(!list_skb->head_frag);
>>> +
>>> + page =
>>> virt_to_head_page(list_skb->head);
>>> + if (!head_frag) {
>>> + head_frag =
>>> kmalloc(sizeof(skb_frag_t),
>>> +
>>> GFP_KERNEL);
>>> + if (!head_frag)
>>> + goto err;
>>> + }
>>
>>
>> Please no memory allocation. I just meant you could allocate it on the
>> stack later.
>>
>>> + head_frag->page.p = page;
>>> + head_frag->page_offset =
>>> list_skb->data -
>>> + (unsigned char
>>> *)page_address(page);
>>> + head_frag->size =
>>> skb_headlen(list_skb);
>>> + /* set i = -1 so we will pick
>>> head_frag
>>> + * instead of
>>> skb_shinfo(list_skb)->frags
>>> + * when i == -1.
>>> + */
>>> + i = -1;
>>> + }
>>
>>
>> So it took me a bit to pick up on the fact that line below wasn't
>> removed. So we are basically trying to do this all in one pass now. Do
>> I have that right?
>>
>> One thing you could look at doing to save yourself the extra "if"
>> later would be to pull frag pointer before you go through skb_headlen
>> check above. Then if you are going to use a head_frag you could just
>> do a "i--; frag--;" combination just to rewind and make the room for
>> the increment to come later. That way you don't have an invalid frag
>> pointer floating around. That way you only have to do this once
>> instead of having to do a conditional check per fragment.
>
>
> Right. This indeed make code more cleaner.
>
>>
>>> nfrags = skb_shinfo(list_skb)->nr_frags;
>>> - frag = skb_shinfo(list_skb)->frags;
>>
>>
>> This patch might be more readable if you were to just insert the
>> skb_headlen() bits down here and left the i=0 through frag = .. in one
>> piece.
>
>
> Right. Will implement as suggested.
>
>>
>>> - frag_skb = list_skb;
>>> -
>>> - BUG_ON(!nfrags);
>>> -
>>> - if (skb_orphan_frags(frag_skb,
>>> GFP_ATOMIC) ||
>>> - skb_zerocopy_clone(nskb, frag_skb,
>>> - GFP_ATOMIC))
>>> - goto err;
>>> + if (nfrags) {
>>> + frag =
>>> skb_shinfo(list_skb)->frags;
>>> + frag_skb = list_skb;
>>> +
>>> + if (skb_orphan_frags(frag_skb,
>>> GFP_ATOMIC) ||
>>> + skb_zerocopy_clone(nskb,
>>> frag_skb,
>>> +
>>> GFP_ATOMIC))
>>> + goto err;
>>> + }
>>>
>>> list_skb = list_skb->next;
>>> }
>>> @@ -3689,7 +3709,7 @@ struct sk_buff *skb_segment(struct sk_buff
>>> *head_skb,
>>> goto err;
>>> }
>>>
>>> - *nskb_frag = *frag;
>>> + *nskb_frag = (i == -1) ? *head_frag : *frag;
>>
>>
>> So this would be better as "*nskb_frag = (i < 0) ? head_frag : *frag;".
>
>
> Good suggestion. Will implement as suggested.
>
>
>>
>>> __skb_frag_ref(nskb_frag);
>>> size = skb_frag_size(nskb_frag);
>>>
>>> @@ -3702,7 +3722,8 @@ struct sk_buff *skb_segment(struct sk_buff
>>> *head_skb,
>>>
>>> if (pos + size <= offset + len) {
>>> i++;
>>> - frag++;
>>> + if (i != 0)
>>> + frag++;
>>> pos += size;
>>> } else {
>>> skb_frag_size_sub(nskb_frag, pos + size
>>> - (offset + len));
>>> @@ -3774,10 +3795,12 @@ struct sk_buff *skb_segment(struct sk_buff
>>> *head_skb,
>>> swap(tail->destructor, head_skb->destructor);
>>> swap(tail->sk, head_skb->sk);
>>> }
>>> + kfree(head_frag);
>>> return segs;
>>>
>>> err:
>>> kfree_skb_list(segs);
>>> + kfree(head_frag);
>>> return ERR_PTR(err);
>>> }
>>> EXPORT_SYMBOL_GPL(skb_segment);
>>> --
>>> 2.9.5
>>>
>
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