On 3/20/18 11:08 AM, Alexander Duyck wrote:
On Tue, Mar 20, 2018 at 8:55 AM, Yonghong Song <y...@fb.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. A one-element frag array is created for the list_skb head
and processed before list_skb->frags are processed.

Reported-by: Diptanu Gon Choudhury <dipt...@fb.com>
Signed-off-by: Yonghong Song <y...@fb.com>
---
  net/core/skbuff.c | 42 +++++++++++++++++++++++++++++-------------
  1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 715c134..0ad4cda 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3475,9 +3475,10 @@ 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;

I would move head_frag down into the while loop. No point in making it
global to this function and eating up the extra stack space if you
don't need it.

We need head_frag declared outside the main segmentation loop.
Its value may survive across different main loop iterations.
However, I see your point to save the stack space.
Will use a pointer here and do allocation on demand instead.


         unsigned int mss = skb_shinfo(head_skb)->gso_size;
         unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
+       struct sk_buff *check_list_skb = list_skb;

This seems like a waste of a pointer. You can probably just repurpose
nfrags to achieve the same purpose you are achieving below. Note that
nfrags is a signed int and only needing to store up to only 18 or so
total frags so we can probably just reserve a nfrags value of -1 to
indicate that we are using the header frag.

Just did some prototyping. Indeed, we could reserve the i = -1 to
indicate the special head_frag.


         struct sk_buff *frag_skb = head_skb;
         unsigned int offset = doffset;
         unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
@@ -3590,6 +3591,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                         nskb = skb_clone(list_skb, GFP_ATOMIC);
                         list_skb = list_skb->next;
+                       check_list_skb = list_skb;

                         if (unlikely(!nskb))
                                 goto err;

If my understanding is correct then this is unneeded if you just
change how you use i and nfrags.

Right.


@@ -3664,21 +3666,35 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,

                 while (pos < offset + len) {

You could probably just declare here since this would be more local to
where it is actually used and the fact is it can be overwritten with
each iteration of the loop anyway so there is no need to reserve the
space before you get here.

The actual space can be allocated once and reused later. But the space
needs to be preserved across main segmentation loop.


                         if (i >= nfrags) {
-                               BUG_ON(skb_headlen(list_skb));
-
-                               i = 0;
-                               nfrags = skb_shinfo(list_skb)->nr_frags;
-                               frag = skb_shinfo(list_skb)->frags;
-                               frag_skb = list_skb;
+                               if (skb_headlen(list_skb) && check_list_skb == 
list_skb) {
+                                       struct page *page;
+
+                                       BUG_ON(!list_skb->head_frag);
+
+                                       i = 0;
+                                       nfrags = 1;
+                                       page = 
virt_to_head_page(list_skb->head);
+                                       head_frag.page.p = page;
+                                       head_frag.page_offset = list_skb->data -
+                                               (unsigned char 
*)page_address(page);
+                                       head_frag.size = skb_headlen(list_skb);
+                                       frag = &head_frag;
+                                       check_list_skb = list_skb->next;

The whole need for check_list_skb can be worked around if we take
advantage of the fact that i and nfrags are both integers so we could
use -1 for both to indicate we are processing the head frag. The only
bit that gets messy is the fact that we have to add special handling
for the case where skb_shinfo(list_skb)->nr_frags is 0. If that is the
case we would have to set nfrags to 0 and bump the list_skb =
list_skb->next so that we avoid trying to pull frags from an otherwise
empty buffer.

Right. if nr_frags = 0, we do need to avoid pulling frags.

Thanks for the review, will send out v3 shortly.


+                               } else {
+                                       i = 0;
+                                       nfrags = skb_shinfo(list_skb)->nr_frags;
+                                       frag = skb_shinfo(list_skb)->frags;
+                                       frag_skb = list_skb;

-                               BUG_ON(!nfrags);
+                                       BUG_ON(!nfrags);

-                               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
-                                   skb_zerocopy_clone(nskb, frag_skb,
-                                                      GFP_ATOMIC))
-                                       goto err;
+                                       if (skb_orphan_frags(frag_skb, 
GFP_ATOMIC) ||
+                                           skb_zerocopy_clone(nskb, frag_skb, 
GFP_ATOMIC))
+                                               goto err;

-                               list_skb = list_skb->next;
+                                       list_skb = list_skb->next;
+                                       check_list_skb = list_skb;
+                               }
                         }

                         if (unlikely(skb_shinfo(nskb)->nr_frags >=
--
2.9.5

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