CVE-2022-48744 net/mlx5e: Avoid field-overflowing memcpy()

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: Avoid field-overflowing memcpy()

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).