[BUG] tiocsti() NULL dereference if ld->ops->receive_buf==NULL

[Jann Horn]

Hi!

When a line discipline doesn't have a ->receive_buf handler, tiocsti()
attempts to call a NULL pointer. Both tty_n_tracesink and
spk_ttyio_ldisc_ops don't have such a handler.

To reproduce, build a kernel with CONFIG_SPEAKUP=y and
CONFIG_SPEAKUP_SYNTH_SOFT=y, set speakup.synth=soft in the kernel
command line, and run the following code as root:

=================
#include <fcntl.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <err.h>

void do_ioctl(int fd, unsigned long cmd, void *arg) {
  int res = ioctl(fd, cmd, arg);
  if (res < 0) {
    perror("ioctl");
  } else {
    printf("result %d\n", res);
  }
}

int main(void) {
  int fd = open("/dev/tty1", O_RDWR);
  if (fd == -1) err(1, "open");

  int disc = 26/*N_SPEAKUP*/;
  do_ioctl(fd, TIOCSETD, &disc);

  char c = 'A';
  do_ioctl(fd, TIOCSTI, &c);

  return 0;
}
=================

You should get this splat:
=================
[  124.609859] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000000
[  124.613510] #PF error: [INSTR]
[  124.614545] PGD 0 P4D 0
[  124.615249] Oops: 0010 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN
[...]
[  124.620994] RIP: 0010:          (null)
[  124.622016] Code: Bad RIP value.
[  124.622884] RSP: 0018:ffff8881d6c37ae0 EFLAGS: 00010246
[...]
[  124.643743] Call Trace:
[  124.644428]  ? tty_ioctl+0x5ec/0xbd0
[  124.645404]  ? tty_vhangup+0x20/0x20
[  124.646380]  ? preempt_count_add+0x8d/0xe0
[  124.647476]  ? _raw_spin_lock_irqsave+0x9a/0xf0
[  124.648702]  ? _raw_read_lock_irqsave+0x60/0x60
[  124.649966]  ? __wake_up_common+0x4b/0x1d0
[  124.651075]  ? preempt_count_sub+0x14/0xc0
[  124.652180]  ? _raw_spin_unlock_irqrestore+0x20/0x40
[  124.653508]  ? __wake_up_common_lock+0xd7/0x130
[  124.654740]  ? __wake_up_common+0x1d0/0x1d0
[  124.655870]  ? mutex_trylock+0x5f/0x90
[  124.656897]  ? ldsem_up_read+0x13/0x40
[  124.657940]  ? tty_write+0x2c7/0x450
[  124.658902]  ? n_tty_open+0xd0/0xd0
[  124.659868]  ? __vfs_write+0xc9/0x3b0
[  124.660876]  ? __anon_vma_prepare+0x159/0x240
[  124.662057]  ? preempt_count_add+0x8d/0xe0
[  124.663170]  ? do_vfs_ioctl+0x134/0x8f0
[  124.664220]  ? fsnotify+0x5ba/0x5e0
[  124.665169]  ? __lru_cache_add+0xc4/0x100
[  124.666255]  ? ioctl_preallocate+0x140/0x140
[  124.667383]  ? __fsnotify_inode_delete+0x20/0x20
[  124.668640]  ? vfs_write+0x120/0x230
[  124.669613]  ? ksys_ioctl+0x70/0x80
[  124.670570]  ? __x64_sys_ioctl+0x3d/0x50
[  124.671634]  ? do_syscall_64+0x73/0x160
[  124.672690]  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  124.674099] Modules linked in: btrfs xor zstd_compress raid6_pq
[  124.676617] CR2: 0000000000000000
[  124.677523] ---[ end trace c5f555a92fe90edc ]---
[  124.678753] RIP: 0010:          (null)
[  124.679762] Code: Bad RIP value.
[...]
=================

I'm not familiar with the details of the TTY subsystem, so I'm not
sure how this should be fixed.