net: strparser: partially revert "strparser: Call skb_unclone conditionally"

This reverts the first part of commit 4e485d06bb ("strparser: Call
skb_unclone conditionally").  To build a message with multiple
fragments we need our own root of frag_list.  We can't simply
use the frag_list of orig_skb, because it will lead to linking
all orig_skbs together creating very long frag chains, and causing
stack overflow on kfree_skb() (which is called recursively on
the frag_lists).

BUG: stack guard page was hit at 00000000d40fad41 (stack is 0000000029dde9f4..000000008cce03d5)
kernel stack overflow (double-fault): 0000 [#1] PREEMPT SMP
RIP: 0010:free_one_page+0x2b/0x490

Call Trace:
  __free_pages_ok+0x143/0x2c0
  skb_release_data+0x8e/0x140
  ? skb_release_data+0xad/0x140
  kfree_skb+0x32/0xb0

  [...]

  skb_release_data+0xad/0x140
  ? skb_release_data+0xad/0x140
  kfree_skb+0x32/0xb0
  skb_release_data+0xad/0x140
  ? skb_release_data+0xad/0x140
  kfree_skb+0x32/0xb0
  skb_release_data+0xad/0x140
  ? skb_release_data+0xad/0x140
  kfree_skb+0x32/0xb0
  skb_release_data+0xad/0x140
  ? skb_release_data+0xad/0x140
  kfree_skb+0x32/0xb0
  skb_release_data+0xad/0x140
  __kfree_skb+0xe/0x20
  tcp_disconnect+0xd6/0x4d0
  tcp_close+0xf4/0x430
  ? tcp_check_oom+0xf0/0xf0
  tls_sk_proto_close+0xe4/0x1e0 [tls]
  inet_release+0x36/0x60
  __sock_release+0x37/0xa0
  sock_close+0x11/0x20
  __fput+0xa2/0x1d0
  task_work_run+0x89/0xb0
  exit_to_usermode_loop+0x9a/0xa0
  do_syscall_64+0xc0/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Let's leave the second unclone conditional, as I'm not entirely
sure what is its purpose :)

Fixes: 4e485d06bb ("strparser: Call skb_unclone conditionally")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/strparser/strparser.c

@@ -140,13 +140,11 @@ static int __strp_recv(read_descriptor_t *desc, struct sk_buff *orig_skb,
 			/* We are going to append to the frags_list of head.
 			 * Need to unshare the frag_list.
 			 */
-			if (skb_has_frag_list(head)) {
+			err = skb_unclone(head, GFP_ATOMIC);
-				err = skb_unclone(head, GFP_ATOMIC);
+			if (err) {
-				if (err) {
+				STRP_STATS_INCR(strp->stats.mem_fail);
-					STRP_STATS_INCR(strp->stats.mem_fail);
+				desc->error = err;
-					desc->error = err;
+				return 0;
-					return 0;
-				}
 			}
 
 			if (unlikely(skb_shinfo(head)->frag_list)) {