Re: [PATCH v2 RESEND] net/mlx5e: Avoid field-overflowing memcpy()

[Saeed Mahameed]

On 24 Jan 09:20, Kees Cook wrote:
> 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>
> ---
> Since this results in no binary differences, I will carry this in my tree
> unless someone else wants to pick it up. It's one of the last remaining
> clean-ups needed for the next step in memcpy() hardening.

applied to net-next-mlx5.

Thanks,
Saeed