[PATCH 2/3] wifi: iwlwifi: mld: fix TSO segmentation explosion when AMSDU is disabled

[Cole Leavitt <cole@unwrap.rs>]

When the TLC notification disables AMSDU for a TID, the MLD driver sets
max_tid_amsdu_len to the sentinel value 1. The TSO segmentation path in
iwl_mld_tx_tso_segment() checks for zero but not for this sentinel,
allowing it to reach the num_subframes calculation:

  num_subframes = (max_tid_amsdu_len + pad) / (subf_len + pad)
                = (1 + 2) / (1534 + 2) = 0

This zero propagates to iwl_tx_tso_segment() which sets:

  gso_size = num_subframes * mss = 0

Calling skb_gso_segment() with gso_size=0 creates over 32000 tiny
segments from a single GSO skb. This floods the TX ring with ~1024
micro-frames (the rest are purged), creating a massive burst of TX
completion events that can lead to memory corruption and a subsequent
use-after-free in TCP's retransmit queue (refcount underflow in
tcp_shifted_skb, NULL deref in tcp_rack_detect_loss).

The MVM driver is immune because it checks mvmsta->amsdu_enabled before
reaching the num_subframes calculation. The MLD driver has no equivalent
bitmap check and relies solely on max_tid_amsdu_len, which does not
catch the sentinel value.

Fix this by detecting the sentinel value (max_tid_amsdu_len == 1) at the
existing check and falling back to non-AMSDU TSO segmentation. Also add
a WARN_ON_ONCE guard after the num_subframes division as defense-in-depth
to catch any future code paths that produce zero through a different
mechanism.

Suggested-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Fixes: d1e879ec600f ("wifi: iwlwifi: add iwlmld sub-driver")
Signed-off-by: Cole Leavitt <cole@unwrap.rs>
---
 drivers/net/wireless/intel/iwlwifi/mld/tx.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/intel/iwlwifi/mld/tx.c b/drivers/net/wireless/intel/iwlwifi/mld/tx.c
index e341d12e5233..8af58aabcd68 100644
--- a/drivers/net/wireless/intel/iwlwifi/mld/tx.c
+++ b/drivers/net/wireless/intel/iwlwifi/mld/tx.c
@@ -823,7 +823,7 @@ static int iwl_mld_tx_tso_segment(struct iwl_mld *mld, struct sk_buff *skb,
 		return -EINVAL;
 
 	max_tid_amsdu_len = sta->cur->max_tid_amsdu_len[tid];
-	if (!max_tid_amsdu_len)
+	if (!max_tid_amsdu_len || max_tid_amsdu_len == 1)
 		return iwl_tx_tso_segment(skb, 1, netdev_flags, mpdus_skbs);
 
 	/* Sub frame header + SNAP + IP header + TCP header + MSS */
@@ -835,6 +835,9 @@ static int iwl_mld_tx_tso_segment(struct iwl_mld *mld, struct sk_buff *skb,
 	 */
 	num_subframes = (max_tid_amsdu_len + pad) / (subf_len + pad);
 
+	if (WARN_ON_ONCE(!num_subframes))
+		return iwl_tx_tso_segment(skb, 1, netdev_flags, mpdus_skbs);
+
 	if (sta->max_amsdu_subframes &&
 	    num_subframes > sta->max_amsdu_subframes)
 		num_subframes = sta->max_amsdu_subframes;
-- 
2.52.0