According to ieee80211_s1g_optional_len(), it can be clearly seen that the
maximum size of variable is 4 and it is an array. Based on the above, the
parsing of the frame control field and optional field is optimized.

Fixes: 1e1f706fc2ce ("wifi: cfg80211/mac80211: correctly parse S1G beacon optional elements")
Reported-by: syzbot+6554b492c7008bcd3385@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6554b492c7008bcd3385
Tested-by: syzbot+6554b492c7008bcd3385@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 include/linux/ieee80211.h | 2 +-
 net/mac80211/mlme.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
index ce377f7fb912..556ce95e0b0f 100644
--- a/include/linux/ieee80211.h
+++ b/include/linux/ieee80211.h
@@ -1278,7 +1278,7 @@ struct ieee80211_ext {
 			u8 sa[ETH_ALEN];
 			__le32 timestamp;
 			u8 change_seq;
-			u8 variable[0];
+			u8 variable[4];
 		} __packed s1g_beacon;
 	} u;
 } __packed __aligned(2);
diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
index 2d46d4af60d7..fb7bf95ee87b 100644
--- a/net/mac80211/mlme.c
+++ b/net/mac80211/mlme.c
@@ -7222,7 +7222,7 @@ static void ieee80211_rx_mgmt_beacon(struct ieee80211_link_data *link,
 	if (ieee80211_is_s1g_beacon(mgmt->frame_control)) {
 		struct ieee80211_ext *ext = (void *) mgmt;
 		variable = ext->u.s1g_beacon.variable +
-			   ieee80211_s1g_optional_len(ext->frame_control);
+			   (ieee80211_s1g_optional_len(ext->frame_control) - 1);
 	}
 
 	baselen = (u8 *) variable - (u8 *) mgmt;
-- 
2.43.0

On Tue, Jun 17, 2025 at 12:41:33PM +0800, Edward Adam Davis wrote:
> According to ieee80211_s1g_optional_len(), it can be clearly seen that the
> maximum size of variable is 4 and it is an array. Based on the above, the
> parsing of the frame control field and optional field is optimized.

Hi,

This is incorrect according to IEEE80211-2024 9.3.4.3. In addition, the 
undefined behaviour reported by the bot due to using zero length arrays
rather then variable length arrays already has a patch submitted by
Johanes - please see:

Link: https://patchwork.kernel.org/project/linux-wireless/patch/20250614003037.a3e82e882251.I2e8b58e56ff2a9f8b06c66f036578b7c1d4e4685@changeid/

lachlan

On Tue, 2025-06-17 at 12:41 +0800, Edward Adam Davis wrote:
> According to ieee80211_s1g_optional_len(), it can be clearly seen that the
> maximum size of variable is 4 and it is an array. Based on the above, the
> parsing of the frame control field and optional field is optimized.
> 
> Fixes: 1e1f706fc2ce ("wifi: cfg80211/mac80211: correctly parse S1G beacon optional elements")
> Reported-by: syzbot+6554b492c7008bcd3385@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=6554b492c7008bcd3385
> Tested-by: syzbot+6554b492c7008bcd3385@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
>  include/linux/ieee80211.h | 2 +-
>  net/mac80211/mlme.c       | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
> index ce377f7fb912..556ce95e0b0f 100644
> --- a/include/linux/ieee80211.h
> +++ b/include/linux/ieee80211.h
> @@ -1278,7 +1278,7 @@ struct ieee80211_ext {
>  			u8 sa[ETH_ALEN];
>  			__le32 timestamp;
>  			u8 change_seq;
> -			u8 variable[0];
> +			u8 variable[4];

That's incorrect when those fields aren't present, and will result in
wrong sizeof(). I believe the correct fix is one I sent before, to just
make it []:

https://lore.kernel.org/linux-wireless/20250614003037.a3e82e882251.I2e8b58e56ff2a9f8b06c66f036578b7c1d4e4685@changeid/

johannes