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KASAN: use-after-free Read in fbcon_cursor

Status: fixed on 2020/09/30 06:27
Reported-by: syzbot+164d60ee54cdb30597a9@syzkaller.appspotmail.com
Fix commit: 770adb5d2b8e fbcon: remove soft scrollback code
First crash: 1805d, last: 1545d
Fix bisection: fixed by (bisect log) :
commit 770adb5d2b8ebe94a92e4c9510f4f2517f4204eb
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon Sep 7 18:45:27 2020 +0000

  fbcon: remove soft scrollback code

  
▶ ▼ Similar bugs (2)
Kernel Title Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-4.14 KASAN: use-after-free Read in fbcon_cursor syz inconclusive 3 1628d 1805d 0/1 upstream: reported syz repro on 2019/12/15 04:16
upstream KASAN: use-after-free Read in fbcon_cursor fbdev syz done error 10 1560d 1804d 15/28 fixed on 2020/09/25 01:17
▶ ▼ Fix bisection attempts (8)
Created Duration User Patch Repo Result
2020/09/30 00:57 3h27m bisect fix linux-4.19.y OK (1) job log
2020/08/30 21:23 28m bisect fix linux-4.19.y OK (0) job log log
2020/07/31 20:54 28m bisect fix linux-4.19.y OK (0) job log log
2020/07/01 20:28 25m bisect fix linux-4.19.y OK (0) job log log
2020/05/11 03:10 26m bisect fix linux-4.19.y OK (0) job log log
2020/04/11 02:40 29m bisect fix linux-4.19.y OK (0) job log log
2020/03/12 02:13 27m bisect fix linux-4.19.y OK (0) job log log
2020/01/14 03:48 25m bisect fix linux-4.19.y OK (0) job log log

Sample crash report:
audit: type=1400 audit(1576381391.863:38): avc:  denied  { associate } for  pid=7812 comm="syz-executor.0" name="syz0" scontext=unconfined_u:object_r:unlabeled_t:s0 tcontext=system_u:object_r:unlabeled_t:s0 tclass=filesystem permissive=1
==================================================================
BUG: KASAN: use-after-free in fbcon_cursor+0x686/0x7b0 drivers/video/fbdev/core/fbcon.c:1347
Read of size 2 at addr ffff88809fedd44c by task syz-executor.0/8323

CPU: 0 PID: 8323 Comm: syz-executor.0 Not tainted 4.19.89-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x197/0x210 lib/dump_stack.c:118
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report mm/kasan/report.c:412 [inline]
 kasan_report.cold+0x8c/0x2ba mm/kasan/report.c:396
 __asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:431
 fbcon_cursor+0x686/0x7b0 drivers/video/fbdev/core/fbcon.c:1347
 fbcon_scrolldelta+0x675/0x1330 drivers/video/fbdev/core/fbcon.c:2855
 fbcon_set_origin+0x43/0x50 drivers/video/fbdev/core/fbcon.c:2906
 set_origin+0x341/0x440 drivers/tty/vt/vt.c:919
 vc_do_resize+0xacc/0x14a0 drivers/tty/vt/vt.c:1267
 vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
 vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
 tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:501 [inline]
 do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
 __do_sys_ioctl fs/ioctl.c:712 [inline]
 __se_sys_ioctl fs/ioctl.c:710 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
 do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45a909
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f77cb9d3c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a909
RDX: 0000000020000000 RSI: 0000000000005609 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f77cb9d46d4
R13: 00000000004c7009 R14: 00000000004dd670 R15: 00000000ffffffff

Allocated by task 7820:
 save_stack+0x45/0xd0 mm/kasan/kasan.c:448
 set_track mm/kasan/kasan.c:460 [inline]
 kasan_kmalloc mm/kasan/kasan.c:553 [inline]
 kasan_kmalloc+0xce/0xf0 mm/kasan/kasan.c:531
 __do_kmalloc mm/slab.c:3727 [inline]
 __kmalloc+0x15d/0x750 mm/slab.c:3736
 kmalloc include/linux/slab.h:520 [inline]
 kzalloc include/linux/slab.h:709 [inline]
 vc_do_resize+0x262/0x14a0 drivers/tty/vt/vt.c:1190
 vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
 vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
 tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:501 [inline]
 do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
 __do_sys_ioctl fs/ioctl.c:712 [inline]
 __se_sys_ioctl fs/ioctl.c:710 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
 do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 8323:
 save_stack+0x45/0xd0 mm/kasan/kasan.c:448
 set_track mm/kasan/kasan.c:460 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
 __cache_free mm/slab.c:3503 [inline]
 kfree+0xcf/0x220 mm/slab.c:3822
 vc_do_resize+0xa69/0x14a0 drivers/tty/vt/vt.c:1264
 vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
 vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
 tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:501 [inline]
 do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
 __do_sys_ioctl fs/ioctl.c:712 [inline]
 __se_sys_ioctl fs/ioctl.c:710 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
 do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88809fedd440
 which belongs to the cache kmalloc-32 of size 32
The buggy address is located 12 bytes inside of
 32-byte region [ffff88809fedd440, ffff88809fedd460)
The buggy address belongs to the page:
page:ffffea00027fb740 count:1 mapcount:0 mapping:ffff88812c31c1c0 index:0xffff88809feddfc1
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffffea0002818d48 ffffea00027f48c8 ffff88812c31c1c0
raw: ffff88809feddfc1 ffff88809fedd000 000000010000003f 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88809fedd300: fb fb fb fb fc fc fc fc 00 00 01 fc fc fc fc fc
 ffff88809fedd380: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
>ffff88809fedd400: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
                                              ^
 ffff88809fedd480: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
 ffff88809fedd500: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
==================================================================

Crashes (4):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2019/12/15 03:46 linux-4.19.y 312017a460d5 eef6e580 .config console log report syz ci2-linux-4-19
2020/06/01 19:31 linux-4.19.y 2d16cf4817bc a0331e89 .config console log report ci2-linux-4-19
2020/05/27 06:24 linux-4.19.y 1bab61d3e8cd 9072c126 .config console log report ci2-linux-4-19
2020/02/10 19:58 linux-4.19.y b499cf4b3a90 d9e55b05 .config console log report ci2-linux-4-19
* Struck through repros no longer work on HEAD.