ppp: fix memory leak in pad_compress_skb

JIRA: https://issues.redhat.com/browse/RHEL-115585
Upstream Status: linux.git

commit 4844123
Author: Qingfang Deng <dqfext@gmail.com>
Date:   Wed Sep 3 18:07:26 2025 +0800

    ppp: fix memory leak in pad_compress_skb

    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: Guillaume Nault <gnault@redhat.com>

Author: Guillaume Nault

drivers/net/ppp/ppp_generic.c

@@ -1749,7 +1749,6 @@ 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;
 	}
@@ -1851,9 +1850,10 @@ 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;
 	}
 
 	/*