[PATCH 2/2] mac80211: make ieee80211_tx_info padding explicit

[PATCH 2/2] mac80211: make ieee80211_tx_info padding explicit

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

While looking at a bug, I got rather confused by the layout of the
'status' field in ieee80211_tx_info. Apparently, the intention is that
status_driver_data[] is used for driver specific data, and fills up the
size of the union to 40 bytes, just like the other ones.

This is indeed what actually happens, but only because of the
combination of two mistakes:

 - "void *status_driver_data[18 / sizeof(void *)];" is intended
   to be 18 bytes long but is actually two bytes shorter because of
   rounding-down in the division, to a multiple of the pointer
   size (4 bytes or 8 bytes).

 - The other fields combined are intended to be 22 bytes long, but
   are actually 24 bytes because of padding in front of the
   unaligned tx_time member, and in front of the pointer array.

The two mistakes cancel out. so the size ends up fine, but it seems
more helpful to make this explicit, by having a multiple of 8 bytes
in the size calculation and explicitly describing the padding.

Fixes: ea5907db2a9cc ("mac80211: fix struct ieee80211_tx_info size")
Fixes: 02219b3abca59 ("mac80211: add WMM admission control support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/net/mac80211.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 3a8a2d2c58c38..ca4dc8a14f1bb 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -1192,9 +1192,11 @@ struct ieee80211_tx_info {
 			u8 ampdu_ack_len;
 			u8 ampdu_len;
 			u8 antenna;
+			u8 pad;
 			u16 tx_time;
 			u8 flags;
-			void *status_driver_data[18 / sizeof(void *)];
+			u8 pad2;
+			void *status_driver_data[16 / sizeof(void *)];
 		} status;
 		struct {
 			struct ieee80211_tx_rate driver_rates[
-- 
2.39.2

Re: [PATCH 2/2] mac80211: make ieee80211_tx_info padding explicit

[Kees Cook]

On Fri, Jun 23, 2023 at 05:24:00PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> While looking at a bug, I got rather confused by the layout of the
> 'status' field in ieee80211_tx_info. Apparently, the intention is that
> status_driver_data[] is used for driver specific data, and fills up the
> size of the union to 40 bytes, just like the other ones.
> 
> This is indeed what actually happens, but only because of the
> combination of two mistakes:
> 
>  - "void *status_driver_data[18 / sizeof(void *)];" is intended
>    to be 18 bytes long but is actually two bytes shorter because of
>    rounding-down in the division, to a multiple of the pointer
>    size (4 bytes or 8 bytes).
> 
>  - The other fields combined are intended to be 22 bytes long, but
>    are actually 24 bytes because of padding in front of the
>    unaligned tx_time member, and in front of the pointer array.
> 
> The two mistakes cancel out. so the size ends up fine, but it seems
> more helpful to make this explicit, by having a multiple of 8 bytes
> in the size calculation and explicitly describing the padding.
> 
> Fixes: ea5907db2a9cc ("mac80211: fix struct ieee80211_tx_info size")
> Fixes: 02219b3abca59 ("mac80211: add WMM admission control support")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  include/net/mac80211.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/mac80211.h b/include/net/mac80211.h
> index 3a8a2d2c58c38..ca4dc8a14f1bb 100644
> --- a/include/net/mac80211.h
> +++ b/include/net/mac80211.h
> @@ -1192,9 +1192,11 @@ struct ieee80211_tx_info {
>  			u8 ampdu_ack_len;
>  			u8 ampdu_len;
>  			u8 antenna;
> +			u8 pad;
>  			u16 tx_time;
>  			u8 flags;
> -			void *status_driver_data[18 / sizeof(void *)];
> +			u8 pad2;
> +			void *status_driver_data[16 / sizeof(void *)];
>  		} status;

pahole agrees with your assessment. :)

struct ieee80211_tx_info {
	...
        union {
		...
                struct {
                        struct ieee80211_tx_rate rates[4]; /*     8    12 */
                        s32        ack_signal;           /*    20     4 */
                        u8         ampdu_ack_len;        /*    24     1 */
                        u8         ampdu_len;            /*    25     1 */
                        u8         antenna;              /*    26     1 */

                        /* XXX 1 byte hole, try to pack */

                        u16        tx_time;              /*    28     2 */
                        u8         flags;                /*    30     1 */

                        /* XXX 1 byte hole, try to pack */

                        void *     status_driver_data[2]; /*    32    16 */
                } status;                                /*     8    40 */
                struct {
                        struct ieee80211_tx_rate driver_rates[4]; /*     8    12 */
                        u8         pad[4];               /*    20     4 */
                        void *     rate_driver_data[3];  /*    24    24 */
                };                                       /*     8    40 */
                void *             driver_data[5];       /*     8    40 */
        };                                               /*     8    40 */
	...
};

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook