[PATCH bpf v3 1/2] 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.

A stream parser is only meant to measure the next message, not to modify
the packet. Reject a parser whose program can change packet data
(prog->aux->changes_pkt_data) at attach time. The check is shared by
sock_map_prog_update() and sock_map_link_update_prog(), which between them
cover prog attach, link create and link update. Verdict programs are
unaffected and may still modify the skb.

Fixes: 8a31db561566 ("bpf: add access to sock fields and pkt data from sk_skb programs")
Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
---
 net/core/sock_map.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 99e3789492a0..c60ba6d292f9 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -1515,6 +1515,17 @@ static int sock_map_prog_link_lookup(struct bpf_map *map, struct bpf_prog ***ppr
 	return 0;
 }
 
+static int sock_map_prog_attach_check(enum bpf_attach_type attach_type,
+				      struct bpf_prog *prog)
+{
+	/* A stream parser must not modify the skb, only measure it. */
+	if (prog && attach_type == BPF_SK_SKB_STREAM_PARSER &&
+	    prog->aux->changes_pkt_data)
+		return -EINVAL;
+
+	return 0;
+}
+
 /* Handle the following four cases:
  * prog_attach: prog != NULL, old == NULL, link == NULL
  * prog_detach: prog == NULL, old != NULL, link == NULL
@@ -1533,6 +1544,10 @@ static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
 	if (ret)
 		return ret;
 
+	ret = sock_map_prog_attach_check(which, prog);
+	if (ret)
+		return ret;
+
 	/* for prog_attach/prog_detach/link_attach, return error if a bpf_link
 	 * exists for that prog.
 	 */
@@ -1776,6 +1791,11 @@ static int sock_map_link_update_prog(struct bpf_link *link,
 		ret = -EINVAL;
 		goto out;
 	}
+
+	ret = sock_map_prog_attach_check(link->attach_type, prog);
+	if (ret)
+		goto out;
+
 	if (!sockmap_link->map) {
 		ret = -ENOLINK;
 		goto out;
-- 
2.43.0