Subject: Re: [PATCH bpf] bpf: Fix NULL pointer dereference in bpf_skb_fib_lookup()

From: Paul Chaignon <[email-masked]>
Date: Mon, 20 Apr 2026 12:41:38 +0200

On Sun, Apr 19, 2026 at 10:01:32AM -0700, Weiming Shi wrote:
> When tot_len is not provided by the user, bpf_skb_fib_lookup()
> resolves the FIB result's output device via dev_get_by_index_rcu()
> to check skb forwardability and fill in mtu_result. The returned
> pointer is dereferenced without a NULL check. If the device is
> concurrently unregistered, dev_get_by_index_rcu() returns NULL and
> is_skb_forwardable() crashes at dev->flags:
> 
>  KASAN: null-ptr-deref in range
>   [0x00000000000000b0-0x00000000000000b7]
>  Call Trace:
>   is_skb_forwardable (include/linux/netdevice.h:4365)
>   bpf_skb_fib_lookup (net/core/filter.c:6446)
>   bpf_prog_test_run_skb (net/bpf/test_run.c)
>   __sys_bpf (kernel/bpf/syscall.c)
> 
> Add the missing NULL check, returning -ENODEV to be consistent
> with how bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup() handle
> the same condition.
> 
> Fixes: e1850ea9bd9e ("bpf: bpf_fib_lookup return MTU value as output when looked up")
> Reported-by: Xiang Mei <[email-masked]>
> Signed-off-by: Weiming Shi <[email-masked]>
> ---
>  net/core/filter.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 78b548158fb0..3e56b567bd18 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6450,6 +6450,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
>  		 * against MTU of FIB lookup resulting net_device
>  		 */
>  		dev = dev_get_by_index_rcu(net, params->ifindex);
> +		if (!dev)
> +			return -ENODEV;

The bug and its fix make sense to me. Given the race, it looks difficult
to write a selftest for this. The condition might be worth an unlikely()
as done in bpf_ipv{4,6}_fib_lookup() above.

Acked-by: Paul Chaignon <[email-masked]>

>  		if (!is_skb_forwardable(dev, skb))
>  			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
>  
> -- 
> 2.43.0
> 
>