ppp: fix memory leak in pad_compress_skb

[ Upstream commit 4844123fe0b853a4982c02666cb3fd863d701d50 ]

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: b3f9b92 ("[PPP]: add PPP MPPE encryption module")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://patch.msgid.link/20250903100726.269839-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 631fc8ab5beb9e0ec8651fb9875b9a968e7b4ae4)

Author: LGA1150

drivers/net/ppp/ppp_generic.c

@@ -1753,7 +1753,6 @@ 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;
 	}
@@ -1855,9 +1854,10 @@ 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;
 	}
 
 	/*