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SubjectRe: [RESEND 2/2] sctp: hold cached endpoints to prevent possible UAF
On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:
> 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.
>
> 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 identify where the bug was introduced? Fixes tag would
be good to have here.

You should squash the two patches together.

> 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;
> }
>

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