[PATCH net] ppp: fix memory leak in pad_compress_skb

[PATCH net] ppp: fix memory leak in pad_compress_skb

[Qingfang Deng]

If alloc_skb() fails in pad_compress_skb(), it returns NULL without
releasing the old skb. The caller does:

    skb = pad_compress_skb(ppp, skb);
    if (!skb)
        goto drop;

drop:
    kfree_skb(skb);

When pad_compress_skb() returns NULL, the reference to the old skb is
lost and kfree_skb(skb) ends up doing nothing, leading to a memory leak.

Align pad_compress_skb() semantics with realloc(): only free the old
skb if allocation and compression succeed.  At the call site, use the
new_skb variable so the original skb is not lost when pad_compress_skb()
fails.

Fixes: b3f9b92a6ec1 ("[PPP]: add PPP MPPE encryption module")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
---
 drivers/net/ppp/ppp_generic.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 65795d099166..f9f0f16c41d1 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1744,7 +1744,6 @@ pad_compress_skb(struct ppp *ppp, struct sk_buff *skb)
 		 */
 		if (net_ratelimit())
 			netdev_err(ppp->dev, "ppp: compressor dropped pkt\n");
-		kfree_skb(skb);
 		consume_skb(new_skb);
 		new_skb = NULL;
 	}
@@ -1845,9 +1844,10 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 					   "down - pkt dropped.\n");
 			goto drop;
 		}
-		skb = pad_compress_skb(ppp, skb);
-		if (!skb)
+		new_skb = pad_compress_skb(ppp, skb);
+		if (!new_skb)
 			goto drop;
+		skb = new_skb;
 	}
 
 	/*
-- 
2.43.0

Re: [PATCH net] ppp: fix memory leak in pad_compress_skb

[Eric Dumazet]

On Wed, Sep 3, 2025 at 3:07 AM Qingfang Deng <dqfext@gmail.com> wrote:
>
> If alloc_skb() fails in pad_compress_skb(), it returns NULL without
> releasing the old skb. The caller does:
>
>     skb = pad_compress_skb(ppp, skb);
>     if (!skb)
>         goto drop;
>
> drop:
>     kfree_skb(skb);
>
> When pad_compress_skb() returns NULL, the reference to the old skb is
> lost and kfree_skb(skb) ends up doing nothing, leading to a memory leak.
>
> Align pad_compress_skb() semantics with realloc(): only free the old
> skb if allocation and compression succeed.  At the call site, use the
> new_skb variable so the original skb is not lost when pad_compress_skb()
> fails.
>
> Fixes: b3f9b92a6ec1 ("[PPP]: add PPP MPPE encryption module")
> Signed-off-by: Qingfang Deng <dqfext@gmail.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net] ppp: fix memory leak in pad_compress_skb

[Yue Haibing]

On 2025/9/3 18:07, Qingfang Deng wrote:
> If alloc_skb() fails in pad_compress_skb(), it returns NULL without
> releasing the old skb. The caller does:
> 
>     skb = pad_compress_skb(ppp, skb);
>     if (!skb)
>         goto drop;
> 
> drop:
>     kfree_skb(skb);
> 
> When pad_compress_skb() returns NULL, the reference to the old skb is
> lost and kfree_skb(skb) ends up doing nothing, leading to a memory leak.
> 
> Align pad_compress_skb() semantics with realloc(): only free the old
> skb if allocation and compression succeed.  At the call site, use the
> new_skb variable so the original skb is not lost when pad_compress_skb()
> fails.
> 
> Fixes: b3f9b92a6ec1 ("[PPP]: add PPP MPPE encryption module")
> Signed-off-by: Qingfang Deng <dqfext@gmail.com>
> ---
>  drivers/net/ppp/ppp_generic.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 65795d099166..f9f0f16c41d1 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -1744,7 +1744,6 @@ pad_compress_skb(struct ppp *ppp, struct sk_buff *skb)
>  		 */
>  		if (net_ratelimit())
>  			netdev_err(ppp->dev, "ppp: compressor dropped pkt\n");
> -		kfree_skb(skb);
>  		consume_skb(new_skb);
>  		new_skb = NULL;
>  	}
> @@ -1845,9 +1844,10 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
>  					   "down - pkt dropped.\n");
>  			goto drop;
>  		}
> -		skb = pad_compress_skb(ppp, skb);
> -		if (!skb)
> +		new_skb = pad_compress_skb(ppp, skb);
> +		if (!new_skb)
>  			goto drop;
> +		skb = new_skb;
>  	}
>  
>  	/*

Reviewed-by: Yue Haibing <yuehaibing@huawei.com>

Re: [PATCH net] ppp: fix memory leak in pad_compress_skb

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed,  3 Sep 2025 18:07:26 +0800 you wrote:
> If alloc_skb() fails in pad_compress_skb(), it returns NULL without
> releasing the old skb. The caller does:
> 
>     skb = pad_compress_skb(ppp, skb);
>     if (!skb)
>         goto drop;
> 
> [...]

Here is the summary with links:
  - [net] ppp: fix memory leak in pad_compress_skb
    https://git.kernel.org/netdev/net/c/4844123fe0b8

You are awesome, thank you!
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