[PATCH] amd-xgbe: set skb to NULL after freeing it

[PATCH] amd-xgbe: set skb to NULL after freeing it

[Pan Bian]

The buffer skb is freed via dev_kfree_skb in a loop. skb may be used
again in the next iteration, resulting in a use-after-free bug. To fix
this, the patch set skb to NULL after dev_kfree_skb(skb).

Signed-off-by: Pan Bian <bianpan2016@163.com>
---
 drivers/net/ethernet/amd/xgbe/xgbe-drv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
index 0cc911f..ac6b82d 100644
--- a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
@@ -2754,6 +2754,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
 				netif_err(pdata, rx_err, netdev,
 					  "error in received packet\n");
 			dev_kfree_skb(skb);
+			skb = NULL;
 			goto next_packet;
 		}
 
@@ -2806,6 +2807,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
 			netif_err(pdata, rx_err, netdev,
 				  "packet length exceeds configured MTU\n");
 			dev_kfree_skb(skb);
+			skb = NULL;
 			goto next_packet;
 		}
 
-- 
2.7.4

Re: [PATCH] amd-xgbe: set skb to NULL after freeing it

[David Miller]

From: Pan Bian <bianpan2016@163.com>
Date: Wed, 28 Nov 2018 16:09:45 +0800

> @@ -2754,6 +2754,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  				netif_err(pdata, rx_err, netdev,
>  					  "error in received packet\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
> @@ -2806,6 +2807,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  			netif_err(pdata, rx_err, netdev,
>  				  "packet length exceeds configured MTU\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
> -- 
> 2.7.4

If either of these two cases are executed, it means that it must be
the case that:

	!last || context_next

is FALSE, therefore the conditional in the tail code of this function:

	/* Check if we need to save state before leaving */
	if (received && (!last || context_next)) {
		rdata = XGBE_GET_DESC_DATA(ring, ring->cur);
		rdata->state_saved = 1;
		rdata->state.skb = skb;
		rdata->state.len = len;
		rdata->state.error = error;
	}

will not trigger, and therefore we will not record the free'd SKB.

And if we iterate the loop again, it is guaranteed that 'skb' will
be set to a new packet or NULL.

So there is no problem here.

Re: [PATCH] amd-xgbe: set skb to NULL after freeing it

[PanBian]

On Wed, Nov 28, 2018 at 10:49:31AM -0800, David Miller wrote:
> From: Pan Bian <bianpan2016@163.com>
> Date: Wed, 28 Nov 2018 16:09:45 +0800
> 
> If either of these two cases are executed, it means that it must be
> the case that:
> 
> 	!last || context_next
> 
> is FALSE, therefore the conditional in the tail code of this function:
> 
> 	/* Check if we need to save state before leaving */
> 	if (received && (!last || context_next)) {
> 		rdata = XGBE_GET_DESC_DATA(ring, ring->cur);
> 		rdata->state_saved = 1;
> 		rdata->state.skb = skb;
> 		rdata->state.len = len;
> 		rdata->state.error = error;
> 	}
> 
> will not trigger, and therefore we will not record the free'd SKB.
> 
> And if we iterate the loop again, it is guaranteed that 'skb' will
> be set to a new packet or NULL.
> 
> So there is no problem here.

Got it. Sorry for my misunderstanding. Thank you and Tom for the 
detailed explanations.

Pan