[PATCH net-next v4 04/15] net: skbuff: fail segmentation when a segment can't be checksummed

[Josef Bacik <josef@xxxxxxxxxxxxxx>]

When the device can't checksum the protocol, skb_segment() checksums
each segment with skb_checksum().  If the segment has unreadable frags,
or its lengths don't add up, skb_checksum() returns 0 and the segment
goes out with a checksum that doesn't cover its payload.

Devmem TCP doesn't get that far today.  Its skbs take SKBFL_SHARED_FRAG
from the zerocopy uarg, so skb_segment() tries to linearize the segment
first, and that fails with -ENOMEM for unreadable frags.  No other path
is known to bring an unreadable segment here either.

Use skb_checksum_full() and fail the segmentation with -EFAULT, so that
the checksum doesn't depend on that flag.

Found by the Sashiko AI review of an earlier fix for the same path with
SG off.

Link: 
https://lore.kernel.org/all/20261009-b4-skb-segment-nosg-unreadable-v1-1-7e2334a55fe8@xxxxxxxxxxxxxx/
Reviewed-by: Fernando Fernandez Mancera <fmancera@xxxxxxx>
Acked-by: Mina Almasry <almasrymina@xxxxxxxxxx>
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 net/core/skbuff.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 0082b12544cf..43bfaa010db7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5294,9 +5294,13 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
                        if (!nskb->remcsum_offload)
                                nskb->ip_summed = CHECKSUM_NONE;
-                       SKB_GSO_CB(nskb)->csum =
-                               skb_checksum(nskb, doffset,
-                                            nskb->len - doffset, 0);
+                       SKB_GSO_CB(nskb)->csum = 0;
+                       if (skb_checksum_full(nskb, doffset,
+                                             nskb->len - doffset,
+                                             &SKB_GSO_CB(nskb)->csum)) {
+                               err = -EFAULT;
+                               goto err;
+                       }
                        SKB_GSO_CB(nskb)->csum_start =
                                skb_headroom(nskb) + doffset;
                }

-- 
2.55.0