On 2021/4/14 18:48, Paolo Abeni wrote:
> the commit 1ddc3229ad3c ("skbuff: remove some unnecessary operation
> in skb_segment_list()") introduces an issue very similar to the
> one already fixed by commit 53475c5dd856 ("net: fix use-after-free when
> UDP GRO with shared fraglist").
>
> If the GSO skb goes though skb_clone() and pskb_expand_head() before
> entering skb_segment_list(), the latter will unshare the frag_list
> skbs and will release the old list. With the reverted commit in place,
> when skb_segment_list() completes, skb->next points to the just
> released list, and later on the kernel will hit UaF.
In that case, is "nskb->next = list_skb" needed before jumpping to
error when __skb_linearize() fails? As there is "nskb->next = list_skb"
before jumpping to error handling when skb_clone() fails.
The inconsistency above is the reason I sent the reverted patch:)
>
> Note that since commit e0e3070a9bc9 ("udp: properly complete L4 GRO
> over UDP tunnel packet") the critical scenario can be reproduced also
> receiving UDP over vxlan traffic with:
>
> NIC (NETIF_F_GRO_FRAGLIST enabled) -> vxlan -> UDP sink
>
> Attaching a packet socket to the NIC will cause skb_clone() and the
> tunnel decapsulation will call pskb_expand_head().
>
> Fixes: 1ddc3229ad3c ("skbuff: remove some unnecessary operation in
> skb_segment_list()")
> Signed-off-by: Paolo Abeni <pab...@redhat.com>
> ---
> net/core/skbuff.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 3ad9e8425ab2..14010c0eec48 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3773,13 +3773,13 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> unsigned int tnl_hlen = skb_tnl_header_len(skb);
> unsigned int delta_truesize = 0;
> unsigned int delta_len = 0;
> + struct sk_buff *tail = NULL;
> struct sk_buff *nskb, *tmp;
> int err;
>
> skb_push(skb, -skb_network_offset(skb) + offset);
>
> skb_shinfo(skb)->frag_list = NULL;
> - skb->next = list_skb;
>
> do {
> nskb = list_skb;
> @@ -3797,8 +3797,17 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> }
> }
>
> - if (unlikely(err))
> + if (!tail)
> + skb->next = nskb;
> + else
> + tail->next = nskb;
> +
> + if (unlikely(err)) {
> + nskb->next = list_skb;
> goto err_linearize;
> + }
> +
> + tail = nskb;
>
> delta_len += nskb->len;
> delta_truesize += nskb->truesize;
> @@ -3825,7 +3834,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>
> skb_gso_reset(skb);
>
> - skb->prev = nskb;
> + skb->prev = tail;
>
> if (skb_needs_linearize(skb, features) &&
> __skb_linearize(skb))
>
