[PATCH] wifi: mt76: fix potential tx_retries underflow

[Lorenzo Bianconi]

> When FIELD_GET returns 0 for the retry count, subtracting 1 causes
> an unsigned integer underflow, resulting in tx_retries becoming a
> very large value (0xFFFFFFFF for u32 or 255 for u8).
> 
> Fix by checking if count is non-zero before subtracting 1.
> 
> Signed-off-by: Ryder Lee <ryder.lee at mediatek.com>

I guess we need proper Fixes tags here. Moreover, if you split this patch
in three separated patches you will make life easier for guys that do
backports :)

> ---
>  drivers/net/wireless/mediatek/mt76/mt7915/mac.c | 10 +++++-----
>  drivers/net/wireless/mediatek/mt76/mt7921/mac.c |  5 +++--
>  drivers/net/wireless/mediatek/mt76/mt7925/mac.c |  5 +++--
>  drivers/net/wireless/mediatek/mt76/mt7996/mac.c |  6 +++---
>  4 files changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
> index cec2c4208..334c19ab2 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
> @@ -912,16 +912,16 @@ mt7915_mac_tx_free(struct mt7915_dev *dev, void *data, int len)
>  		}
>  
>  		if (!mtk_wed_device_active(&mdev->mmio.wed) && wcid) {
> -			u32 tx_retries = 0, tx_failed = 0;
> +			u32 tx_retries = 0, tx_failed = 0, count;
>  
>  			if (v3 && (info & MT_TX_FREE_MPDU_HEADER_V3)) {
> -				tx_retries =
> -					FIELD_GET(MT_TX_FREE_COUNT_V3, info) - 1;
> +				count = FIELD_GET(MT_TX_FREE_COUNT_V3, info);
> +				tx_retries = count ? count - 1 : 0;

nit: I think it is more readable if you use a int for tx_retries and do
something like:
				tx_retries = max_t(int, tx_retries, 0);

This is valid even for below chunks.

Regards,
Lorenzo

>  				tx_failed = tx_retries +
>  					!!FIELD_GET(MT_TX_FREE_STAT_V3, info);
>  			} else if (!v3 && (info & MT_TX_FREE_MPDU_HEADER)) {
> -				tx_retries =
> -					FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
> +				count = FIELD_GET(MT_TX_FREE_COUNT, info);
> +				tx_retries = count ? count - 1 : 0;
>  				tx_failed = tx_retries +
>  					!!FIELD_GET(MT_TX_FREE_STAT, info);
>  			}
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> index 03b4960db..668bfa195 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> @@ -530,8 +530,9 @@ static void mt7921_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
>  		stat = FIELD_GET(MT_TX_FREE_STATUS, info);
>  
>  		if (wcid) {
> -			wcid->stats.tx_retries +=
> -				FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
> +			u32 count = FIELD_GET(MT_TX_FREE_COUNT, info);
> +
> +			wcid->stats.tx_retries += count ? count - 1 : 0;
>  			wcid->stats.tx_failed += !!stat;
>  		}
>  
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7925/mac.c b/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
> index c47bd812b..c56a9e530 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
> @@ -1141,8 +1141,9 @@ mt7925_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
>  
>  		if (info & MT_TXFREE_INFO_HEADER) {
>  			if (wcid) {
> -				wcid->stats.tx_retries +=
> -					FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
> +				u32 count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
> +
> +				wcid->stats.tx_retries += count ? count - 1 : 0;
>  				wcid->stats.tx_failed +=
>  					!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
>  			}
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> index a59c14c8f..3fad977ba 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> @@ -1361,13 +1361,13 @@ mt7996_mac_tx_free(struct mt7996_dev *dev, void *data, int len)
>  				cur_info++;
>  			continue;
>  		} else if (info & MT_TXFREE_INFO_HEADER) {
> -			u32 tx_retries = 0, tx_failed = 0;
> +			u32 tx_retries = 0, tx_failed = 0, count;
>  
>  			if (!wcid)
>  				continue;
>  
> -			tx_retries =
> -				FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
> +			count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
> +			tx_retries = count ? count - 1 : 0;
>  			tx_failed = tx_retries +
>  				!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
>  
> -- 
> 2.45.2
> 
> 
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