general protection fault in fbcon_modechanged

[syzbot <syzbot+d396a0ae28ab4507bc89@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    e42617b8 Linux 5.5-rc1
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15229c2ae00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3754e2c78c1adb82
dashboard link: https://syzkaller.appspot.com/bug?extid=d396a0ae28ab4507bc89
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d396a0ae28ab4507bc89@syzkaller.appspotmail.com

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 24279 Comm: syz-executor.0 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:fbcon_modechanged+0xeb/0x790 drivers/video/fbdev/core/fbcon.c:2967
Code: 8d 14 ff 4c 8b 2c d5 80 19 10 8c 48 ba 00 00 00 00 00 fc ff df 49 8d  
85 7c 03 00 00 48 89 c1 48 89 45 c8 83 e0 07 48 c1 e9 03 <0f> b6 14 11 38  
c2 7f 08 84 d2 0f 85 61 05 00 00 41 0f b6 b5 7c 03
RSP: 0018:ffffc90002b27828 EFLAGS: 00010207
RAX: 0000000000000004 RBX: ffff888218cc2400 RCX: 000000000000006f
RDX: dffffc0000000000 RSI: ffffffff83b22859 RDI: ffffffff8c102af0
RBP: ffffc90002b27878 R08: ffff888093236040 R09: ffffed1015d2703d
R10: ffffed1015d2703c R11: ffff8880ae9381e3 R12: ffff888218d25000
R13: 0000000000000000 R14: ffff888218d25468 R15: 000000000000003e
FS:  0000000000000000(0000) GS:ffff8880ae900000(0063) knlGS:00000000f5de3b40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 000000000812a0f0 CR3: 000000008f8fb000 CR4: 00000000001426e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  fbcon_update_vcs+0x42/0x50 drivers/video/fbdev/core/fbcon.c:3038
  fb_set_var+0xb32/0xdd0 drivers/video/fbdev/core/fbmem.c:1051
  do_fb_ioctl+0x390/0x7d0 drivers/video/fbdev/core/fbmem.c:1104
  fb_compat_ioctl+0x305/0xc50 drivers/video/fbdev/core/fbmem.c:1310
  __do_compat_sys_ioctl fs/compat_ioctl.c:214 [inline]
  __se_compat_sys_ioctl fs/compat_ioctl.c:142 [inline]
  __ia32_compat_sys_ioctl+0x233/0x610 fs/compat_ioctl.c:142
  do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
  do_fast_syscall_32+0x27b/0xe16 arch/x86/entry/common.c:408
  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7fe7a39
Code: 00 00 00 89 d3 5b 5e 5f 5d c3 b8 80 96 98 00 eb c4 8b 04 24 c3 8b 1c  
24 c3 8b 34 24 c3 8b 3c 24 c3 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90  
90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000f5de30cc EFLAGS: 00000296 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000004601
RDX: 0000000020000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
---[ end trace 500a2104f8ca30f5 ]---
RIP: 0010:fbcon_modechanged+0xeb/0x790 drivers/video/fbdev/core/fbcon.c:2967
Code: 8d 14 ff 4c 8b 2c d5 80 19 10 8c 48 ba 00 00 00 00 00 fc ff df 49 8d  
85 7c 03 00 00 48 89 c1 48 89 45 c8 83 e0 07 48 c1 e9 03 <0f> b6 14 11 38  
c2 7f 08 84 d2 0f 85 61 05 00 00 41 0f b6 b5 7c 03
RSP: 0018:ffffc90002b27828 EFLAGS: 00010207
RAX: 0000000000000004 RBX: ffff888218cc2400 RCX: 000000000000006f
RDX: dffffc0000000000 RSI: ffffffff83b22859 RDI: ffffffff8c102af0
RBP: ffffc90002b27878 R08: ffff888093236040 R09: ffffed1015d2703d
R10: ffffed1015d2703c R11: ffff8880ae9381e3 R12: ffff888218d25000
R13: 0000000000000000 R14: ffff888218d25468 R15: 000000000000003e
FS:  0000000000000000(0000) GS:ffff8880ae900000(0063) knlGS:00000000f5de3b40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000000bf2fd0 CR3: 000000008f8fb000 CR4: 00000000001426e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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