[PATCH bpf v2] bpf, sockmap: fix use-after-free when the stream parser resizes the skb

[Sechang Lim <rhkrqnwk98@gmail.com>]

sk_psock_strp_parse() runs the BPF_PROG_TYPE_SK_SKB stream-parser program
to find the length of the next message. strparser assembles a message out
of several received skbs by chaining them onto the head's frag_list and
recording where to append the next one in strp->skb_nextp:

	*strp->skb_nextp = skb;
	strp->skb_nextp = &skb->next;

and then calls the parser on the head:

	len = (*strp->cb.parse_msg)(strp, head);

The parser is only meant to inspect the skb, but the program may call
bpf_skb_change_tail() -- or the sibling bpf_skb_pull_data(),
bpf_skb_change_head(), bpf_skb_adjust_room(), all allowed for SK_SKB.
Once the head carries a frag_list these go

	... -> skb_ensure_writable -> pskb_may_pull -> __pskb_pull_tail

and __pskb_pull_tail() frees the frag_list skbs that strparser still
tracks through skb_nextp:

	while ((list = skb_shinfo(skb)->frag_list) != insp) {
		skb_shinfo(skb)->frag_list = list->next;
		consume_skb(list);
	}

strp->skb_nextp now points into a freed sk_buff. The next segment of
the same message arrives in __strp_recv(), which links it with
*strp->skb_nextp = skb, an 8-byte write into the freed skb. The free
and the write happen in different __strp_recv() calls, so the message
has to span at least three segments before it triggers.

  BUG: KASAN: slab-use-after-free in __strp_recv+0x447/0xda0
  Write of size 8 at addr ffff88810db86140 by task repro/349

  Call Trace:
   <IRQ>
   __strp_recv+0x447/0xda0
   __tcp_read_sock+0x13d/0x590
   tcp_bpf_strp_read_sock+0x195/0x320
   strp_data_ready+0x267/0x340
   sk_psock_strp_data_ready+0x1ce/0x350
   tcp_data_queue+0x1364/0x2fd0
   tcp_rcv_established+0xe07/0x1640
   [...]

  Allocated by task 349:
   skb_clone+0x17b/0x210
   __strp_recv+0x2c3/0xda0
   __tcp_read_sock+0x13d/0x590
   [...]

  Freed by task 349:
   kmem_cache_free+0x150/0x570
   __pskb_pull_tail+0x57b/0xc20
   skb_ensure_writable+0x236/0x260
   __bpf_skb_change_tail+0x1d4/0x590
   sk_skb_change_tail+0x2a/0x40
   bpf_prog_1b285dcd6c41373e+0x27/0x30
   bpf_prog_run_pin_on_cpu+0xf3/0x260
   sk_psock_strp_parse+0x118/0x1e0
   __strp_recv+0x4f6/0xda0
   [...]

The same resize also leaves the head's length inconsistent with its
frags, so a later __pskb_pull_tail() can instead hit the
BUG_ON(skb_copy_bits(...)) in net/core/skbuff.c.

Run the parser on a private clone of the head when the message spans more
than one skb and the program can modify the packet
(prog->aux->changes_pkt_data), so a resizing helper can only touch the
clone and strparser's head and skb_nextp stay valid. Single-skb messages
have no frag_list and read-only parsers cannot resize, so both are still
parsed in place. If the clone cannot be allocated, return 0 so the caller
retries on the next read rather than failing the parser.

Fixes: 8a31db561566 ("bpf: add access to sock fields and pkt data from sk_skb programs")
Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
---
v2:
 - clone only when prog->aux->changes_pkt_data (Bobby Eshleman)
 - return 0 on clone failure instead of -ENOMEM (Bobby Eshleman)
 - free the clone with consume_skb() instead of kfree_skb()
 - drop the unrelated guard(rcu)() change (Bobby Eshleman)

v1:
 - https://lore.kernel.org/all/20260609112316.3685738-1-rhkrqnwk98@gmail.com/

 net/core/skmsg.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index e1850caf1a71..501e8cfc7505 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -1149,9 +1149,28 @@ static int sk_psock_strp_parse(struct strparser *strp, struct sk_buff *skb)
 	rcu_read_lock();
 	prog = READ_ONCE(psock->progs.stream_parser);
 	if (likely(prog)) {
-		skb->sk = psock->sk;
-		ret = bpf_prog_run_pin_on_cpu(prog, skb);
-		skb->sk = NULL;
+		struct sk_buff *parse_skb = skb;
+
+		/*
+		 * strparser chains the message skbs through skb->frag_list and
+		 * keeps a pointer into that list in strp->skb_nextp.  The parser
+		 * program may call bpf_skb_change_tail() and friends, which go
+		 * through __pskb_pull_tail() and free the frag_list skbs that
+		 * strparser still tracks.  Run the program on a clone when the head
+		 * has a frag_list and the program can modify the packet, so it
+		 * cannot drop frags strparser owns.
+		 */
+		if (skb_has_frag_list(skb) && prog->aux->changes_pkt_data) {
+			parse_skb = skb_clone(skb, GFP_ATOMIC);
+			if (!parse_skb) {
+				rcu_read_unlock();
+				return 0;
+			}
+		}
+		parse_skb->sk = psock->sk;
+		ret = bpf_prog_run_pin_on_cpu(prog, parse_skb);
+		parse_skb->sk = NULL;
+		if (parse_skb != skb)
+			consume_skb(parse_skb);
 	}
 	rcu_read_unlock();
 	return ret;
-- 
2.43.0