Re: [PATCH net] mpls: reload header after pskb_may_pull()

On Fri, Aug 14, 2026 at 05:54:04PM +0800, Qing Ming wrote:
> mpls_select_multipath() calls mpls_multipath_hash() to choose a nexthop
> when an MPLS route has multiple nexthops.  While walking the MPLS label
> stack, the hash routine caches hdr for the current label.  After finding
> the bottom-of-stack label, it calls pskb_may_pull() before reading the
> inner IP header.
> 
> If an skb is constructed with the inner IP header in nonlinear data and
> insufficient tailroom in the linear head, pskb_may_pull() calls
> pskb_expand_head() to replace the skb head and free the old one.  This
> leaves hdr pointing to freed memory.  The IPv6 path can invalidate hdr
> again when it performs a second pull for the larger header.
> 
> The issue was found through static analysis.  A reproducer sending a legal
> Geneve packet through a bareudp/MPLS multipath setup triggered the same
> KASAN report in 2 of 2 unpatched runs:
> 
>   BUG: KASAN: slab-use-after-free in mpls_select_multipath
>   Read of size 1 at addr ffff88800ecc6e20 by task ksoftirqd/1/23
> 
>   Call Trace:
>    mpls_select_multipath
>    mpls_forward
>    __netif_receive_skb_list_core
>    netif_receive_skb_list_internal
>    napi_complete_done
>    gro_cell_poll
>    __napi_poll
>    net_rx_action
> 
>   Freed by task 23:
>    kfree
>    pskb_expand_head
>    __pskb_pull_tail
>    mpls_select_multipath
> 
> Reload hdr from the current skb head after each successful pull before
> deriving the inner IPv4 or IPv6 header pointer.
> 
> Fixes: 9f427a0e474a ("net: mpls: Fix multipath selection for LSR use case")
> Cc: [email protected]
> Signed-off-by: Qing Ming <[email protected]>

Reviewed-by: Simon Horman <[email protected]>