[PATCH] wifi: ath5k: do not access array OOB

["Jiri Slaby (SUSE)" <jirislaby@kernel.org>]

Vincent reports:
> The ath5k driver seems to do an array-index-out-of-bounds access as
> shown by the UBSAN kernel message:
> UBSAN: array-index-out-of-bounds in drivers/net/wireless/ath/ath5k/base.c:1741:20
> index 4 is out of range for type 'ieee80211_tx_rate [4]'
> ...
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x5d/0x80
>  ubsan_epilogue+0x5/0x2b
>  __ubsan_handle_out_of_bounds.cold+0x46/0x4b
>  ath5k_tasklet_tx+0x4e0/0x560 [ath5k]
>  tasklet_action_common+0xb5/0x1c0

It is real. 'ts->ts_final_idx' can be 3 on 5212, so:
   info->status.rates[ts->ts_final_idx + 1].idx = -1;
with the array defined as:
   struct ieee80211_tx_rate rates[IEEE80211_TX_MAX_RATES];
while the size is:
   #define IEEE80211_TX_MAX_RATES  4
is indeed bogus.

Set this 'idx = -1' sentinel only if the array index is less than the
array size. As mac80211 will not look at rates beyond the size
(IEEE80211_TX_MAX_RATES).

Note: The effect of the OOB write is negligible. It just overwrites the
next member of info->status, i.e. ack_signal.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Reported-by: Vincent Danjean <vdanjean@debian.org>
Link: https://lore.kernel.org/all/aQYUkIaT87ccDCin@eldamar.lan
Closes: https://bugs.debian.org/1119093

---
Cc: 1119093@bugs.debian.org
Cc: Salvatore Bonaccorso <carnil@debian.org>
Cc: Nick Kossifidis <mickflemm@gmail.com>,
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: linux-wireless@vger.kernel.org
---
 drivers/net/wireless/ath/ath5k/base.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/ath/ath5k/base.c b/drivers/net/wireless/ath/ath5k/base.c
index 4d88b02ffa79..917e1b087924 100644
--- a/drivers/net/wireless/ath/ath5k/base.c
+++ b/drivers/net/wireless/ath/ath5k/base.c
@@ -1738,7 +1738,8 @@ ath5k_tx_frame_completed(struct ath5k_hw *ah, struct sk_buff *skb,
 	}
 
 	info->status.rates[ts->ts_final_idx].count = ts->ts_final_retry;
-	info->status.rates[ts->ts_final_idx + 1].idx = -1;
+	if (ts->ts_final_idx + 1 < IEEE80211_TX_MAX_RATES)
+		info->status.rates[ts->ts_final_idx + 1].idx = -1;
 
 	if (unlikely(ts->ts_status)) {
 		ah->stats.ack_fail++;
-- 
2.51.1