general protection fault in spk_ttyio_ldisc_close

[Kyungtae Kim]

We report a bug in linux-4.20: "general protection fault in
spk_ttyio_ldisc_close"

kernel config: https://kt0755.github.io/etc/config_v4.20_stable
repro: https://kt0755.github.io/etc/repro.a670e.c

This occurs when the function kfree is about to execute
(driver/staging/speakup/spk_ttyio.c:68).
Particularly, kfree takes the argument like speakup_tty->disc_data.
But speakup_tty is invalid, so the pointer dereference causes GPF.
At a glance, it seems that speakup_tty was deallocated somewhere ahead of kfree.

=========================================
general protection fault: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 0 PID: 13246 Comm: syz-executor7 Not tainted 4.20.0-rc2 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
RIP: 0010:spk_ttyio_ldisc_close+0x3f/0x80 drivers/staging/speakup/spk_ttyio.c:68
Code: 35 95 8c e8 43 03 60 01 48 8b 1d dc 2e 3f 07 48 ba 00 00 00 00
00 fc ff df 48 8d bb 70 05 00 00 48 89 f8 48 c1 e8 03 48 01 d0 <80> 38
00 75 26 48 8b bb 70 05 00 00 e8 a0 68 4f fa 48 c7 c7 c0 35
RSP: 0018:ffff88810e6f7960 EFLAGS: 00010282
RAX: dffffc00000000ae RBX: 0000000000000000 RCX: 1ffff11021cdef00
RDX: dffffc0000000000 RSI: 0000000000000008 RDI: 0000000000000570
RBP: ffff88810e6f7968 R08: fffffbfff192a6b9 R09: fffffbfff192a6b9
R10: ffff88810e6f7950 R11: fffffbfff192a6b8 R12: ffff8881063ee6b0
R13: ffffffff87471020 R14: ffff8881063eeea8 R15: ffff888116ec7b80
FS:  00007f607f746700(0000) GS:ffff88811a000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000006fb2e0 CR3: 000000010f186000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 tty_ldisc_close drivers/tty/tty_ldisc.c:477 [inline]
 tty_ldisc_kill+0xef/0x1a0 drivers/tty/tty_ldisc.c:623
 tty_ldisc_release+0x111/0x230 drivers/tty/tty_ldisc.c:790
 tty_release_struct+0x1f/0x60 drivers/tty/tty_io.c:1595
 tty_release+0xb80/0x11c0 drivers/tty/tty_io.c:1768
 __fput+0x2b8/0x7a0 fs/file_table.c:278
 ____fput+0x1a/0x20 fs/file_table.c:309
 task_work_run+0x15b/0x1e0 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x8d6/0x30d0 kernel/exit.c:867
 do_group_exit+0x13d/0x370 kernel/exit.c:970
 get_signal+0x6bb/0x1890 kernel/signal.c:2517
 do_signal+0x8c/0x1a10 arch/x86/kernel/signal.c:816
 exit_to_usermode_loop+0x186/0x1d0 arch/x86/entry/common.c:162
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
 do_syscall_64+0x445/0x4f0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4497b9
Code: e8 8c 9f 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48
89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d
01 f0 ff ff 0f 83 9b 6b fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f607f745ce8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 000000000071bf80 RCX: 00000000004497b9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 000000000071bf80
RBP: 000000000071bf80 R08: 0000000000000000 R09: 000000000071bf58
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f607f7469c0 R15: 00007f607f746700
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
=========================================

Thanks,
Kyungtae Kim