[PATCH 2/3] net/mlx5e: Avoid field-overflowing memcpy()

[Kees Cook]

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use flexible arrays instead of zero-element arrays (which look like they
are always overflowing) and split the cross-field memcpy() into two halves
that can be appropriately bounds-checked by the compiler.

We were doing:

	#define ETH_HLEN  14
	#define VLAN_HLEN  4
	...
	#define MLX5E_XDP_MIN_INLINE (ETH_HLEN + VLAN_HLEN)
	...
        struct mlx5e_tx_wqe      *wqe  = mlx5_wq_cyc_get_wqe(wq, pi);
	...
        struct mlx5_wqe_eth_seg  *eseg = &wqe->eth;
        struct mlx5_wqe_data_seg *dseg = wqe->data;
	...
	memcpy(eseg->inline_hdr.start, xdptxd->data, MLX5E_XDP_MIN_INLINE);

target is wqe->eth.inline_hdr.start (which the compiler sees as being
2 bytes in size), but copying 18, intending to write across start
(really vlan_tci, 2 bytes). The remaining 16 bytes get written into
wqe->data[0], covering byte_count (4 bytes), lkey (4 bytes), and addr
(8 bytes).

struct mlx5e_tx_wqe {
        struct mlx5_wqe_ctrl_seg   ctrl;                 /*     0    16 */
        struct mlx5_wqe_eth_seg    eth;                  /*    16    16 */
        struct mlx5_wqe_data_seg   data[];               /*    32     0 */

        /* size: 32, cachelines: 1, members: 3 */
        /* last cacheline: 32 bytes */
};

struct mlx5_wqe_eth_seg {
        u8                         swp_outer_l4_offset;  /*     0     1 */
        u8                         swp_outer_l3_offset;  /*     1     1 */
        u8                         swp_inner_l4_offset;  /*     2     1 */
        u8                         swp_inner_l3_offset;  /*     3     1 */
        u8                         cs_flags;             /*     4     1 */
        u8                         swp_flags;            /*     5     1 */
        __be16                     mss;                  /*     6     2 */
        __be32                     flow_table_metadata;  /*     8     4 */
        union {
                struct {
                        __be16     sz;                   /*    12     2 */
                        u8         start[2];             /*    14     2 */
                } inline_hdr;                            /*    12     4 */
                struct {
                        __be16     type;                 /*    12     2 */
                        __be16     vlan_tci;             /*    14     2 */
                } insert;                                /*    12     4 */
                __be32             trailer;              /*    12     4 */
        };                                               /*    12     4 */

        /* size: 16, cachelines: 1, members: 9 */
        /* last cacheline: 16 bytes */
};

struct mlx5_wqe_data_seg {
        __be32                     byte_count;           /*     0     4 */
        __be32                     lkey;                 /*     4     4 */
        __be64                     addr;                 /*     8     8 */

        /* size: 16, cachelines: 1, members: 3 */
        /* last cacheline: 16 bytes */
};

So, split the memcpy() so the compiler can reason about the buffer
sizes.

"pahole" shows no size nor member offset changes to struct mlx5e_tx_wqe
nor struct mlx5e_umr_wqe. "objdump -d" shows no meaningful object
code changes (i.e. only source line number induced differences and
optimizations).

Cc: Saeed Mahameed <saeedm@xxxxxxxxxx>
Cc: Leon Romanovsky <leon@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-rdma@xxxxxxxxxxxxxxx
Cc: bpf@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/net/ethernet/mellanox/mlx5/core/en.h     | 4 ++--
 drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c | 4 +++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
index 4f6897c1ea8d..8997476c20cc 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
@@ -200,7 +200,7 @@ static inline int mlx5e_get_max_num_channels(struct mlx5_core_dev *mdev)
 struct mlx5e_tx_wqe {
 	struct mlx5_wqe_ctrl_seg ctrl;
 	struct mlx5_wqe_eth_seg  eth;
-	struct mlx5_wqe_data_seg data[0];
+	struct mlx5_wqe_data_seg data[];
 };
 
 struct mlx5e_rx_wqe_ll {
@@ -216,7 +216,7 @@ struct mlx5e_umr_wqe {
 	struct mlx5_wqe_ctrl_seg       ctrl;
 	struct mlx5_wqe_umr_ctrl_seg   uctrl;
 	struct mlx5_mkey_seg           mkc;
-	struct mlx5_mtt                inline_mtts[0];
+	struct mlx5_mtt                inline_mtts[];
 };
 
 extern const char mlx5e_self_tests[][ETH_GSTRING_LEN];
diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
index 2f0df5cc1a2d..efae2444c26f 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
@@ -341,8 +341,10 @@ mlx5e_xmit_xdp_frame(struct mlx5e_xdpsq *sq, struct mlx5e_xmit_data *xdptxd,
 
 	/* copy the inline part if required */
 	if (sq->min_inline_mode != MLX5_INLINE_MODE_NONE) {
-		memcpy(eseg->inline_hdr.start, xdptxd->data, MLX5E_XDP_MIN_INLINE);
+		memcpy(eseg->inline_hdr.start, xdptxd->data, sizeof(eseg->inline_hdr.start));
 		eseg->inline_hdr.sz = cpu_to_be16(MLX5E_XDP_MIN_INLINE);
+		memcpy(dseg, xdptxd->data + sizeof(eseg->inline_hdr.start),
+		       MLX5E_XDP_MIN_INLINE - sizeof(eseg->inline_hdr.start));
 		dma_len  -= MLX5E_XDP_MIN_INLINE;
 		dma_addr += MLX5E_XDP_MIN_INLINE;
 		dseg++;
-- 
2.30.2