[RFC - is this a bug?] wifi: ath10k: Asking for some light on this, please :)

[Gustavo A. R. Silva]

[+CC Manikanta Pubbisetty ]

As Johannes[1] pointed out, this `memset()` is probably unnecessary:

./drivers/net/wireless/ath/ath10k/wmi.c:
8920         memset(skb->data, 0, sizeof(*cmd));

However, the same exact issue[2] is present at the line below inside
function `ath10k_wmi_alloc_skb()`:

drivers/net/wireless/ath/ath10k/wmi.c:
1799         memset(skb->data, 0, round_len);


Thanks
--
Gustavo

[1] https://lore.kernel.org/linux-hardening/26b15f4702cef17fe70b496a62f03735874bd16a.camel@sipsolutions.net/
[2] https://lore.kernel.org/linux-hardening/07e9bb04-f9fc-46d5-bfb9-a00a63a707c0@embeddedor.com/

On 10/24/23 13:50, Gustavo A. R. Silva wrote:
> Hi all,
> 
> While working on tranforming one-element array `peer_chan_list` in
> `struct wmi_tdls_peer_capabilities` into a flex-array member
> 
> 7187 struct wmi_tdls_peer_capabilities {
> ...
> 7199         struct wmi_channel peer_chan_list[1];
> 7200 } __packed;
> 
> the following line caught my attention:
> 
> ./drivers/net/wireless/ath/ath10k/wmi.c:
> 8920         memset(skb->data, 0, sizeof(*cmd));
> 
> Notice that before the flex-array transformation, we are zeroing 128
> bytes in `skb->data` because `sizeof(*cmd) == 128`, see below:
> 
> $ pahole -C wmi_10_4_tdls_peer_update_cmd drivers/net/wireless/ath/ath10k/wmi.o
> struct wmi_10_4_tdls_peer_update_cmd {
>      __le32                     vdev_id;              /*     0     4 */
>      struct wmi_mac_addr        peer_macaddr;         /*     4     8 */
>      __le32                     peer_state;           /*    12     4 */
>      __le32                     reserved[4];          /*    16    16 */
>      struct wmi_tdls_peer_capabilities peer_capab;    /*    32    96 */
> 
>      /* size: 128, cachelines: 2, members: 5 */
> };
> 
> So, after the flex-array transformation (and the necessary adjustments
> to a few other lines of code) we would be zeroing 104 bytes in
> `skb->data` because `sizeof(*cmd) == 104`, see below:
> 
> $ pahole -C wmi_10_4_tdls_peer_update_cmd drivers/net/wireless/ath/ath10k/wmi.o
> struct wmi_10_4_tdls_peer_update_cmd {
>      __le32                     vdev_id;              /*     0     4 */
>      struct wmi_mac_addr        peer_macaddr;         /*     4     8 */
>      __le32                     peer_state;           /*    12     4 */
>      __le32                     reserved[4];          /*    16    16 */
>      struct wmi_tdls_peer_capabilities peer_capab;    /*    32    72 */
> 
>      /* size: 104, cachelines: 2, members: 5 */
>      /* last cacheline: 40 bytes */
> };
> 
> This difference arises because the size of the element type for the
> `peer_chan_list` array, which is `sizeof(struct wmi_channel) == 24 `
> 
> $ pahole -C wmi_channel drivers/net/wireless/ath/ath10k/wmi.o
> struct wmi_channel {
>      __le32                     mhz;                  /*     0     4 */
>      __le32                     band_center_freq1;    /*     4     4 */
>      __le32                     band_center_freq2;    /*     8     4 */
> 
> [..]
>                                                 /*    20     4 */
> 
>      /* size: 24, cachelines: 1, members: 6 */
>      /* last cacheline: 24 bytes */
> };
> 
> is included in `sizeof(*cmd)` before the transformation.
> 
> So, my question is: do we really need to zero out those extra 24 bytes in
> `skb->data`? or is it rather a bug in the original code?
> 
> Thanks!
> -- 
> Gustavo
> 
>