On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski [off-list ref] wrote:

On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:

quoted

The cause of the resultant dump_stack() reported below is a
dereference of a freed pointer to 'struct sctp_endpoint' in
sctp_sock_dump().

This race condition occurs when a transport is cached into its
associated hash table followed by an endpoint/sock migration to a new
association in sctp_assoc_migrate() prior to their subsequent use in
sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
table calling into sctp_sock_dump() where the dereference occurs.

in sctp_sock_dump():
        struct sock *sk = ep->base.sk;
        ... <--[1]
        lock_sock(sk);

Do you mean in [1], the sk is peeled off and gets freed elsewhere?
if that's true, it's still late to do sock_hold(sk) in your this patch.

I talked with Marcelo about this before, if the possible UAF in [1] exists,
the problem also exists in the main RX path sctp_rcv().

quoted

  BUG: KASAN: use-after-free in sctp_sock_dump+0xa8/0x438 [sctp_diag]
  Call trace:
   dump_backtrace+0x0/0x2dc
   show_stack+0x20/0x2c
   dump_stack+0x120/0x144
   print_address_description+0x80/0x2f4
   __kasan_report+0x174/0x194
   kasan_report+0x10/0x18
   __asan_load8+0x84/0x8c
   sctp_sock_dump+0xa8/0x438 [sctp_diag]
   sctp_for_each_transport+0x1e0/0x26c [sctp]
   sctp_diag_dump+0x180/0x1f0 [sctp_diag]
   inet_diag_dump+0x12c/0x168
   netlink_dump+0x24c/0x5b8
   __netlink_dump_start+0x274/0x2a8
   inet_diag_handler_cmd+0x224/0x274
   sock_diag_rcv_msg+0x21c/0x230
   netlink_rcv_skb+0xe0/0x1bc
   sock_diag_rcv+0x34/0x48
   netlink_unicast+0x3b4/0x430
   netlink_sendmsg+0x4f0/0x574
   sock_write_iter+0x18c/0x1f0
   do_iter_readv_writev+0x230/0x2a8
   do_iter_write+0xc8/0x2b4
   vfs_writev+0xf8/0x184
   do_writev+0xb0/0x1a8
   __arm64_sys_writev+0x4c/0x5c
   el0_svc_common+0x118/0x250
   el0_svc_handler+0x3c/0x9c
   el0_svc+0x8/0xc

To prevent this from happening we need to take a references to the
to-be-used/dereferenced 'struct sock' and 'struct sctp_endpoint's
until such a time when we know it can be safely released.

When KASAN is not enabled, a similar, but slightly different NULL
pointer derefernce crash occurs later along the thread of execution in
inet_sctp_diag_fill() this time.

Are you able to reproduce this issue?

What I'm thinking is to fix it by freeing sk in call_rcu() by
sock_set_flag(sock->sk, SOCK_RCU_FREE),
and add rcu_read_lock() in sctp_sock_dump().

Thanks.

Are you able to identify where the bug was introduced? Fixes tag would
be good to have here.

You should squash the two patches together.

quoted

diff --git a/net/sctp/diag.c b/net/sctp/diag.c
index 760b367644c12..2029b240b6f24 100644
--- a/net/sctp/diag.c
+++ b/net/sctp/diag.c

@@ -301,6 +301,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
      struct sctp_association *assoc;
      int err = 0;

+     sctp_endpoint_hold(ep);
+     sock_hold(sk);
      lock_sock(sk);
      list_for_each_entry(assoc, &ep->asocs, asocs) {
              if (cb->args[4] < cb->args[1])

@@ -341,6 +343,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
      cb->args[4] = 0;
 release:
      release_sock(sk);
+     sock_put(sk);
+     sctp_endpoint_put(ep);
      return err;
 }