[patch 33/47] net: Fix OOPS in skb_seq_read().

[Greg KH]

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Shyam Iyer <shyam_iyer@xxxxxxxx>

[ Upstream commit 71b3346d182355f19509fadb8fe45114a35cc499 ]

It oopsd for me in skb_seq_read. addr2line said it was
linux-2.6/net/core/skbuff.c:2228, which is this line:

	while (st->frag_idx < skb_shinfo(st->cur_skb)->nr_frags) {

I added some printks in there and it looks like we hit this:

        } else if (st->root_skb == st->cur_skb &&
                   skb_shinfo(st->root_skb)->frag_list) {
                 st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
                 st->frag_idx = 0;
                 goto next_skb;
        }

Actually I did some testing and added a few printks and found that the
st->cur_skb->data was 0 and hence the ptr used by iscsi_tcp was null.
This caused the kernel panic.

 	if (abs_offset < block_limit) {
-		*data = st->cur_skb->data + abs_offset;
+		*data = st->cur_skb->data + (abs_offset - st->stepped_offset);

I enabled the debug_tcp and with a few printks found that the code did
not go to the next_skb label and could find that the sequence being
followed was this -

It hit this if condition -

        if (st->cur_skb->next) {
                st->cur_skb = st->cur_skb->next;
                st->frag_idx = 0;
                goto next_skb;

And so, now the st pointer is shifted to the next skb whereas actually
it should have hit the second else if first since the data is in the
frag_list.

        else if (st->root_skb == st->cur_skb &&
                 skb_shinfo(st->root_skb)->frag_list) {
                st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
                goto next_skb;
        }

Reversing the two conditions the attached patch fixes the issue for me
on top of Herbert's patches.

Signed-off-by: Shyam Iyer <shyam_iyer@xxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 net/core/skbuff.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2039,13 +2039,13 @@ next_skb:
 		st->frag_data = NULL;
 	}
 
-	if (st->cur_skb->next) {
-		st->cur_skb = st->cur_skb->next;
+	if (st->root_skb == st->cur_skb &&
+	    skb_shinfo(st->root_skb)->frag_list) {
+		st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
 		st->frag_idx = 0;
 		goto next_skb;
-	} else if (st->root_skb == st->cur_skb &&
-		   skb_shinfo(st->root_skb)->frag_list) {
-		st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
+	} else if (st->cur_skb->next) {
+		st->cur_skb = st->cur_skb->next;
 		st->frag_idx = 0;
 		goto next_skb;
 	}

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