Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup_elem() on an xskmap

[Daniel Borkmann <daniel@iogearbox.net>]

On 06/03/2019 06:38 PM, Jonathan Lemon wrote:
> Currently, the AF_XDP code uses a separate map in order to
> determine if an xsk is bound to a queue.  Instead of doing this,
> have bpf_map_lookup_elem() return the queue_id, as a way of
> indicating that there is a valid entry at the map index.
> 
> Rearrange some xdp_sock members to eliminate structure holes.
> 
> Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
> Acked-by: Song Liu <songliubraving@fb.com>
> Acked-by: Björn Töpel <bjorn.topel@intel.com>
> ---
>  include/net/xdp_sock.h                            |  6 +++---
>  kernel/bpf/verifier.c                             |  6 +++++-
>  kernel/bpf/xskmap.c                               |  4 +++-
>  .../selftests/bpf/verifier/prevent_map_lookup.c   | 15 ---------------
>  4 files changed, 11 insertions(+), 20 deletions(-)
> 
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index d074b6d60f8a..7d84b1da43d2 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -57,12 +57,12 @@ struct xdp_sock {
>  	struct net_device *dev;
>  	struct xdp_umem *umem;
>  	struct list_head flush_node;
> -	u16 queue_id;
> -	struct xsk_queue *tx ____cacheline_aligned_in_smp;
> -	struct list_head list;
> +	u32 queue_id;
>  	bool zc;
>  	/* Protects multiple processes in the control path */
>  	struct mutex mutex;
> +	struct xsk_queue *tx ____cacheline_aligned_in_smp;
> +	struct list_head list;
>  	/* Mutual exclusion of NAPI TX thread and sendmsg error paths
>  	 * in the SKB destructor callback.
>  	 */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2778417e6e0c..91c730f85e92 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2905,10 +2905,14 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>  	 * appear.
>  	 */
>  	case BPF_MAP_TYPE_CPUMAP:
> -	case BPF_MAP_TYPE_XSKMAP:
>  		if (func_id != BPF_FUNC_redirect_map)
>  			goto error;
>  		break;
> +	case BPF_MAP_TYPE_XSKMAP:
> +		if (func_id != BPF_FUNC_redirect_map &&
> +		    func_id != BPF_FUNC_map_lookup_elem)
> +			goto error;
> +		break;
>  	case BPF_MAP_TYPE_ARRAY_OF_MAPS:
>  	case BPF_MAP_TYPE_HASH_OF_MAPS:
>  		if (func_id != BPF_FUNC_map_lookup_elem)
> diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
> index 686d244e798d..249b22089014 100644
> --- a/kernel/bpf/xskmap.c
> +++ b/kernel/bpf/xskmap.c
> @@ -154,7 +154,9 @@ void __xsk_map_flush(struct bpf_map *map)
>  
>  static void *xsk_map_lookup_elem(struct bpf_map *map, void *key)
>  {
> -	return ERR_PTR(-EOPNOTSUPP);
> +	struct xdp_sock *xs = __xsk_map_lookup_elem(map, *(u32 *)key);
> +
> +	return xs ? &xs->queue_id : NULL;
>  }

How do you guarantee that BPf programs don't mess around with the map values
e.g. overriding xs->queue_id from the lookup? This should be read-only map
from BPF program side.

>  static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> diff --git a/tools/testing/selftests/bpf/verifier/prevent_map_lookup.c b/tools/testing/selftests/bpf/verifier/prevent_map_lookup.c
> index bbdba990fefb..da7a4b37cb98 100644
> --- a/tools/testing/selftests/bpf/verifier/prevent_map_lookup.c
> +++ b/tools/testing/selftests/bpf/verifier/prevent_map_lookup.c
> @@ -28,21 +28,6 @@
>  	.errstr = "cannot pass map_type 18 into func bpf_map_lookup_elem",
>  	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
>  },
> -{
> -	"prevent map lookup in xskmap",
> -	.insns = {
> -	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
> -	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
> -	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
> -	BPF_LD_MAP_FD(BPF_REG_1, 0),
> -	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
> -	BPF_EXIT_INSN(),
> -	},
> -	.fixup_map_xskmap = { 3 },
> -	.result = REJECT,
> -	.errstr = "cannot pass map_type 17 into func bpf_map_lookup_elem",
> -	.prog_type = BPF_PROG_TYPE_XDP,
> -},
>  {
>  	"prevent map lookup in stack trace",
>  	.insns = {
>