[PATCH 6.1 261/609] phonet: pep: fix use-after-free in pep_get_sb()

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Breno Leitao <[email protected]>

commit 0f71f852a96af9685858ce59fda34ecbf85c283d upstream.

pep_get_sb() doesn't consider that pskb_may_pull() might have relocated
the skb data, and continue to access the older pointer, causing UAF.

Reproduced under KASAN:

  BUG: KASAN: slab-use-after-free in pep_get_sb+0x234/0x3b0
  Read of size 1 at addr ff11000105510f50 by task repro/157
   pep_get_sb+0x234/0x3b0
   pipe_handler_do_rcv+0x5f7/0xa10
   pep_do_rcv+0x203/0x410
   __sk_receive_skb+0x471/0x4a0
   phonet_rcv+0x5b3/0x6c0
   __netif_receive_skb+0xcc/0x1d0

Refetch the header with skb_header_pointer() after pskb_may_pull(), so
the possibly stale pointer is no longer dereferenced. There are better
ways to solve this, but, this is the less instrusive one.

Fixes: 9641458d3ec4 ("Phonet: Pipe End Point for Phonet Pipes protocol")
Cc: [email protected]
Signed-off-by: Breno Leitao <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 net/phonet/pep.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/net/phonet/pep.c
+++ b/net/phonet/pep.c
@@ -55,6 +55,8 @@ static unsigned char *pep_get_sb(struct
 	ph = skb_header_pointer(skb, 0, 2, &h);
 	if (ph == NULL || ph->sb_len < 2 || !pskb_may_pull(skb, ph->sb_len))
 		return NULL;
+	/* pskb_may_pull() may have reallocated the head; refetch ph. */
+	ph = skb_header_pointer(skb, 0, 2, &h);
 	ph->sb_len -= 2;
 	*ptype = ph->sb_type;
 	*plen = ph->sb_len;