net: Fix use after free in the recvmmsg exit path

The syzkaller fuzzer hit the following use-after-free:

  Call Trace:
   [<ffffffff8175ea0e>] __asan_report_load8_noabort+0x3e/0x40 mm/kasan/report.c:295
   [<ffffffff851cc31a>] __sys_recvmmsg+0x6fa/0x7f0 net/socket.c:2261
   [<     inline     >] SYSC_recvmmsg net/socket.c:2281
   [<ffffffff851cc57f>] SyS_recvmmsg+0x16f/0x180 net/socket.c:2270
   [<ffffffff86332bb6>] entry_SYSCALL_64_fastpath+0x16/0x7a
  arch/x86/entry/entry_64.S:185

And, as Dmitry rightly assessed, that is because we can drop the
reference and then touch it when the underlying recvmsg calls return
some packets and then hit an error, which will make recvmmsg to set
sock->sk->sk_err, oops, fix it.

Reported-and-Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Fixes: a2e2725541fa ("net: Introduce recvmmsg socket syscall")
http://lkml.kernel.org/r/20160122211644.GC2470@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
SYSCALL_DEFINE4(socketpair, int, family, int, type, int, protocol,
		int __user *, usockvec)
{
	struct socket *sock1, *sock2;
	int fd1, fd2, err;
	struct file *newfile1, *newfile2;
	int flags;

	flags = type & ~SOCK_TYPE_MASK;
	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
		return -EINVAL;
	type &= SOCK_TYPE_MASK;

	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;

	/*
	 * Obtain the first socket and check if the underlying protocol
	 * supports the socketpair call.
	 */

	err = sock_create(family, type, protocol, &sock1);
	if (err < 0)
		goto out;

	err = sock_create(family, type, protocol, &sock2);
	if (err < 0)
		goto out_release_1;

	err = sock1->ops->socketpair(sock1, sock2);
	if (err < 0)
		goto out_release_both;

	fd1 = get_unused_fd_flags(flags);
	if (unlikely(fd1 < 0)) {
		err = fd1;
		goto out_release_both;
	}

	fd2 = get_unused_fd_flags(flags);
	if (unlikely(fd2 < 0)) {
		err = fd2;
		goto out_put_unused_1;
	}

	newfile1 = sock_alloc_file(sock1, flags, NULL);
	if (IS_ERR(newfile1)) {
		err = PTR_ERR(newfile1);
		goto out_put_unused_both;
	}

	newfile2 = sock_alloc_file(sock2, flags, NULL);
	if (IS_ERR(newfile2)) {
		err = PTR_ERR(newfile2);
		goto out_fput_1;
	}

	err = put_user(fd1, &usockvec[0]);
	if (err)
		goto out_fput_both;

	err = put_user(fd2, &usockvec[1]);
	if (err)
		goto out_fput_both;

	audit_fd_pair(fd1, fd2);

	fd_install(fd1, newfile1);
	fd_install(fd2, newfile2);
	/* fd1 and fd2 may be already another descriptors.
	 * Not kernel problem.
	 */

	return 0;

out_fput_both:
	fput(newfile2);
	fput(newfile1);
	put_unused_fd(fd2);
	put_unused_fd(fd1);
	goto out;

out_fput_1:
	fput(newfile1);
	put_unused_fd(fd2);
	put_unused_fd(fd1);
	sock_release(sock2);
	goto out;

out_put_unused_both:
	put_unused_fd(fd2);
out_put_unused_1:
	put_unused_fd(fd1);
out_release_both:
	sock_release(sock2);
out_release_1:
	sock_release(sock1);
out:
	return err;
}

SYSCALL_DEFINE4(socketpair, int, family, int, type, int, protocol,
		int __user *, usockvec)
{
	struct socket *sock1, *sock2;
	int fd1, fd2, err;
	struct file *newfile1, *newfile2;
	int flags;

	flags = type & ~SOCK_TYPE_MASK;
	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
		return -EINVAL;
	type &= SOCK_TYPE_MASK;

	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;

	/*
	 * Obtain the first socket and check if the underlying protocol
	 * supports the socketpair call.
	 */

	err = sock_create(family, type, protocol, &sock1);
	if (err < 0)
		goto out;

	err = sock_create(family, type, protocol, &sock2);
	if (err < 0)
		goto out_release_1;

	err = sock1->ops->socketpair(sock1, sock2);
	if (err < 0)
		goto out_release_both;

	fd1 = get_unused_fd_flags(flags);
	if (unlikely(fd1 < 0)) {
		err = fd1;
		goto out_release_both;
	}

	fd2 = get_unused_fd_flags(flags);
	if (unlikely(fd2 < 0)) {
		err = fd2;
		goto out_put_unused_1;
	}

	newfile1 = sock_alloc_file(sock1, flags, NULL);
	if (IS_ERR(newfile1)) {
		err = PTR_ERR(newfile1);
		goto out_put_unused_both;
	}

	newfile2 = sock_alloc_file(sock2, flags, NULL);
	if (IS_ERR(newfile2)) {
		err = PTR_ERR(newfile2);
		goto out_fput_1;
	}

	err = put_user(fd1, &usockvec[0]);
	if (err)
		goto out_fput_both;

	err = put_user(fd2, &usockvec[1]);
	if (err)
		goto out_fput_both;

	audit_fd_pair(fd1, fd2);

	fd_install(fd1, newfile1);
	fd_install(fd2, newfile2);
	/* fd1 and fd2 may be already another descriptors.
	 * Not kernel problem.
	 */

	return 0;

out_fput_both:
	fput(newfile2);
	fput(newfile1);
	put_unused_fd(fd2);
	put_unused_fd(fd1);
	goto out;

out_fput_1:
	fput(newfile1);
	put_unused_fd(fd2);
	put_unused_fd(fd1);
	sock_release(sock2);
	goto out;

out_put_unused_both:
	put_unused_fd(fd2);
out_put_unused_1:
	put_unused_fd(fd1);
out_release_both:
	sock_release(sock2);
out_release_1:
	sock_release(sock1);
out:
	return err;
}
