Re: [PATCH v2 1/3] wifi: mwifiex: Fix tlv_buf_left calculation

[Kees Cook <keescook@chromium.org>]

On Thu, Aug 24, 2023 at 09:06:51PM -0600, Gustavo A. R. Silva wrote:
> In a TLV encoding scheme, the Length part represents the length after
> the header containing the values for type and length. In this case,
> `tlv_len` should be:
> 
> tlv_len == (sizeof(*tlv_rxba) - 1) - sizeof(tlv_rxba->header) + tlv_bitmap_len
> 
> Notice that the `- 1` accounts for the one-element array `bitmap`, which
> 1-byte size is already included in `sizeof(*tlv_rxba)`.
> 
> So, if the above is correct, there is a double-counting of some members
> in `struct mwifiex_ie_types_rxba_sync`, when `tlv_buf_left` and `tmp`
> are calculated:
> 
> 968                 tlv_buf_left -= (sizeof(*tlv_rxba) + tlv_len);
> 969                 tmp = (u8 *)tlv_rxba + tlv_len + sizeof(*tlv_rxba);
> 
> in specific, members:
> 
> drivers/net/wireless/marvell/mwifiex/fw.h:777
>  777         u8 mac[ETH_ALEN];
>  778         u8 tid;
>  779         u8 reserved;
>  780         __le16 seq_num;
>  781         __le16 bitmap_len;
> 
> This is clearly wrong, and affects the subsequent decoding of data in
> `event_buf` through `tlv_rxba`:
> 
> 970                 tlv_rxba = (struct mwifiex_ie_types_rxba_sync *)tmp;
> 
> Fix this by using `sizeof(tlv_rxba->header)` instead of `sizeof(*tlv_rxba)`
> in the calculation of `tlv_buf_left` and `tmp`.
> 
> This results in the following binary differences before/after changes:
> 
> | drivers/net/wireless/marvell/mwifiex/11n_rxreorder.o
> | @@ -4698,11 +4698,11 @@
> |  drivers/net/wireless/marvell/mwifiex/11n_rxreorder.c:968
> |                 tlv_buf_left -= (sizeof(tlv_rxba->header) + tlv_len);
> | -    1da7:      lea    -0x11(%rbx),%edx
> | +    1da7:      lea    -0x4(%rbx),%edx
> |      1daa:      movzwl %bp,%eax
> |  drivers/net/wireless/marvell/mwifiex/11n_rxreorder.c:969
> |                 tmp = (u8 *)tlv_rxba  + sizeof(tlv_rxba->header) + tlv_len;
> | -    1dad:      lea    0x11(%r15,%rbp,1),%r15
> | +    1dad:      lea    0x4(%r15,%rbp,1),%r15
> 
> The above reflects the desired change: avoid counting 13 too many bytes;
> which is the total size of the double-counted members in
> `struct mwifiex_ie_types_rxba_sync`:
> 
> $ pahole -C mwifiex_ie_types_rxba_sync drivers/net/wireless/marvell/mwifiex/11n_rxreorder.o
> struct mwifiex_ie_types_rxba_sync {
> 	struct mwifiex_ie_types_header header;           /*     0     4 */
> 
>      |-----------------------------------------------------------------------
>      |  u8                         mac[6];               /*     4     6 */  |
>      |	u8                         tid;                  /*    10     1 */  |
>      |  u8                         reserved;             /*    11     1 */  |
>      | 	__le16                     seq_num;              /*    12     2 */  |
>      | 	__le16                     bitmap_len;           /*    14     2 */  |
>      |  u8                         bitmap[1];            /*    16     1 */  |
>      |----------------------------------------------------------------------|
> 								  | 13 bytes|
> 								  -----------
> 
> 	/* size: 17, cachelines: 1, members: 7 */
> 	/* last cacheline: 17 bytes */
> } __attribute__((__packed__));
> 
> Fixes: 99ffe72cdae4 ("mwifiex: process rxba_sync event")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Excellent commit log!

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

-- 
Kees Cook