CVE-2025-40350: net/mlx5e: RX, Fix generating skb from non-linear xdp_buff for striding RQ

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: RX, Fix generating skb from non-linear xdpbuff for striding RQ

XDP programs can change the layout of an xdpbuff through bpfxdpadjusttail() and bpfxdpadjusthead(). Therefore, the driver cannot assume the size of the linear data area nor fragments. Fix the bug in mlx5 by generating skb according to xdpbuff after XDP programs run.

Currently, when handling multi-buf XDP, the mlx5 driver assumes the layout of an xdpbuff to be unchanged. That is, the linear data area continues to be empty and fragments remain the same. This may cause the driver to generate erroneous skb or triggering a kernel warning. When an XDP program added linear data through bpfxdpadjusthead(), the linear data will be ignored as mlx5ebuildlinearskb() builds an skb without linear data and then pull data from fragments to fill the linear data area. When an XDP program has shrunk the non-linear data through bpfxdpadjusttail(), the delta passed to pskbpulltail() may exceed the actual nonlinear data size and trigger the BUGON in it.

To fix the issue, first record the original number of fragments. If the number of fragments changes after the XDP program runs, rewind the end fragment pointer by the difference and recalculate the truesize. Then, build the skb with the linear data area matching the xdpbuff. Finally, only pull data in if there is non-linear data and fill the linear part up to 256 bytes.