Re: [PATCH net v3] net: fix memory leak in skb_segment_list for GRO packets

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

mheib@ wrote:
> From: Mohammad Heib <mheib@redhat.com>
> 
> When skb_segment_list() is called during packet forwarding, it handles
> packets that were aggregated by the GRO engine.
> 
> Historically, the segmentation logic in skb_segment_list assumes that
> individual segments are split from a parent SKB and may need to carry
> their own socket memory accounting. Accordingly, the code transfers
> truesize from the parent to the newly created segments.
> 
> Prior to commit ed4cccef64c1 ("gro: fix ownership transfer"), this
> truesize subtraction in skb_segment_list() was valid because fragments
> still carry a reference to the original socket.
> 
> However, commit ed4cccef64c1 ("gro: fix ownership transfer") changed
> this behavior by ensuring that fraglist entries are explicitly
> orphaned (skb->sk = NULL) to prevent illegal orphaning later in the
> stack. This change meant that the entire socket memory charge remained
> with the head SKB, but the corresponding accounting logic in
> skb_segment_list() was never updated.
> 
> As a result, the current code unconditionally adds each fragment's
> truesize to delta_truesize and subtracts it from the parent SKB. Since
> the fragments are no longer charged to the socket, this subtraction
> results in an effective under-count of memory when the head is freed.
> This causes sk_wmem_alloc to remain non-zero, preventing socket
> destruction and leading to a persistent memory leak.
> 
> The leak can be observed via KMEMLEAK when tearing down the networking
> environment:
> 
> unreferenced object 0xffff8881e6eb9100 (size 2048):
>   comm "ping", pid 6720, jiffies 4295492526
>   backtrace:
>     kmem_cache_alloc_noprof+0x5c6/0x800
>     sk_prot_alloc+0x5b/0x220
>     sk_alloc+0x35/0xa00
>     inet6_create.part.0+0x303/0x10d0
>     __sock_create+0x248/0x640
>     __sys_socket+0x11b/0x1d0
> 
> Since skb_segment_list() is exclusively used for SKB_GSO_FRAGLIST
> packets constructed by GRO, the truesize adjustment is removed.
> 
> The call to skb_release_head_state() must be preserved. As documented in
> commit cf673ed0e057 ("net: fix fraglist segmentation reference count
> leak"), it is still required to correctly drop references to SKB
> extensions that may be overwritten during __copy_skb_header().
> 
> Fixes: ed4cccef64c1 ("gro: fix ownership transfer")
> Signed-off-by: Mohammad Heib <mheib@redhat.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>