KASAN: slab-out-of-bounds Read in cw_putcs

Status: moderation: reported on 2026/08/12 00:12
Subsystems: fbdev
Labels: prio:high
[Documentation on labels]
Reported-by: syzbot+9734671a36be0c0ca6f8@syzkaller.appspotmail.com
First crash: 7d20h, last: 7d20h
▶ ▼ ✨ AI Jobs (2)
ID Workflow Result Correct Bug Created Started Finished Revision
343591e8 repro-c Reproduced: ✅ ❓ KASAN: slab-out-of-bounds Read in cw_putcs 2026/08/12 14:11 2026/08/12 14:11 2026/08/12 14:20 a56bc0a5
340573d9 assessment-security DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ✅ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ KASAN: slab-out-of-bounds Read in cw_putcs 2026/08/08 20:49 2026/08/08 20:49 2026/08/08 21:19 5d63f80b
▶ ▼ Last patch testing requests (1)
Created Duration User Patch Repo Result
2026/08/12 14:23 19m nogikh@google.com upstream report job log log

Sample crash report:
==================================================================
BUG: KASAN: slab-out-of-bounds in __fb_pad_aligned_buffer include/linux/fb.h:646 [inline]
BUG: KASAN: slab-out-of-bounds in cw_putcs_aligned drivers/video/fbdev/core/fbcon_cw.c:102 [inline]
BUG: KASAN: slab-out-of-bounds in cw_putcs+0x9ca/0xbb0 drivers/video/fbdev/core/fbcon_cw.c:159
Read of size 1 at addr ffff88802cf7d9e0 by task syz.2.565/7990

CPU: 1 UID: 0 PID: 7990 Comm: syz.2.565 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x13d/0x4b0 mm/kasan/report.c:482
 kasan_report+0xdf/0x1c0 mm/kasan/report.c:595
 __fb_pad_aligned_buffer include/linux/fb.h:646 [inline]
 cw_putcs_aligned drivers/video/fbdev/core/fbcon_cw.c:102 [inline]
 cw_putcs+0x9ca/0xbb0 drivers/video/fbdev/core/fbcon_cw.c:159
 fbcon_putcs+0x3b2/0x480 drivers/video/fbdev/core/fbcon.c:1392
 con_putc+0x144/0x170 drivers/tty/vt/vt.c:330
 hide_softcursor drivers/tty/vt/vt.c:872 [inline]
 hide_cursor+0x1b6/0x230 drivers/tty/vt/vt.c:884
 redraw_screen+0x5d4/0x760 drivers/tty/vt/vt.c:985
 vc_do_resize+0xc3d/0xeb0 drivers/tty/vt/vt.c:1309
 vc_resize include/linux/vt_kern.h:49 [inline]
 fbcon_do_set_font+0x443/0x8c0 drivers/video/fbdev/core/fbcon.c:2435
 fbcon_set_font+0x4be/0x730 drivers/video/fbdev/core/fbcon.c:2518
 con_font_set drivers/tty/vt/vt.c:4992 [inline]
 con_font_op+0x9c0/0x1040 drivers/tty/vt/vt.c:5032
 vt_k_ioctl drivers/tty/vt/vt_ioctl.c:472 [inline]
 vt_ioctl+0xdd1/0x3110 drivers/tty/vt/vt_ioctl.c:743
 tty_ioctl+0x26a/0x1640 drivers/tty/tty_io.c:2792
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6e1e79e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 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 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6e1f679028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f6e1ea25fa0 RCX: 00007f6e1e79e0d9
RDX: 0000200000000000 RSI: 0000000000004b72 RDI: 0000000000000005
RBP: 00007f6e1e835024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6e1ea26038 R14: 00007f6e1ea25fa0 R15: 00007ffc0fa01a68
 </TASK>

Allocated by task 7841:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
 __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
 kasan_kmalloc include/linux/kasan.h:263 [inline]
 __do_kmalloc_node mm/slub.c:5334 [inline]
 __kmalloc_noprof+0x322/0x820 mm/slub.c:5359
 _kmalloc_noprof include/linux/slab.h:992 [inline]
 _kmalloc_array_noprof include/linux/slab.h:1208 [inline]
 font_data_rotate+0x353/0x3b0 lib/fonts/font_rotate.c:231
 fbcon_rotate_font+0x2bf/0x510 drivers/video/fbdev/core/fbcon_rotate.c:33
 fbcon_switch+0xccc/0x19f0 drivers/video/fbdev/core/fbcon.c:2217
 redraw_screen+0x2c1/0x760 drivers/tty/vt/vt.c:994
 vc_do_resize+0xc3d/0xeb0 drivers/tty/vt/vt.c:1309
 vc_resize include/linux/vt_kern.h:49 [inline]
 fbcon_modechanged+0x342/0x6f0 drivers/video/fbdev/core/fbcon.c:2653
 fbcon_set_all_vcs+0x1d6/0x460 drivers/video/fbdev/core/fbcon.c:2700
 fbcon_rotate_all drivers/video/fbdev/core/fbcon.c:254 [inline]
 rotate_all_store+0x31e/0x410 drivers/video/fbdev/core/fbcon.c:3193
 dev_attr_store+0x58/0x80 drivers/base/core.c:2505
 sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:145
 kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:345
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1050 fs/read_write.c:687
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88802cf7c000
 which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 2528 bytes to the right of
 allocated 4096-byte region [ffff88802cf7c000, ffff88802cf7d000)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2cf78
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801bc43040 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800040004 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801bc43040 dead000000000100 dead000000000122
head: 0000000000000000 0000000800040004 00000000f5000000 0000000000000000
head: 00fff00000000003 fffffffffffffe01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd2040(__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5146, tgid 5146 (udevd), ts 21437858675, free_ts 21262142340
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1859
 prep_new_page mm/page_alloc.c:1867 [inline]
 get_page_from_freelist+0xf48/0x3530 mm/page_alloc.c:3946
 __alloc_frozen_pages_noprof+0x299/0x2dc0 mm/page_alloc.c:5304
 alloc_slab_page mm/slub.c:3266 [inline]
 allocate_slab mm/slub.c:3380 [inline]
 new_slab+0xa2/0x640 mm/slub.c:3426
 refill_objects+0xe3/0x410 mm/slub.c:7310
 refill_sheaf mm/slub.c:2804 [inline]
 __pcs_replace_empty_main+0x376/0x680 mm/slub.c:4675
 alloc_from_pcs mm/slub.c:4773 [inline]
 slab_alloc_node mm/slub.c:4905 [inline]
 __do_kmalloc_node mm/slub.c:5333 [inline]
 __kmalloc_noprof+0x66d/0x820 mm/slub.c:5359
 _kmalloc_noprof include/linux/slab.h:992 [inline]
 tomoyo_realpath_from_path+0xb6/0x690 security/tomoyo/realpath.c:251
 tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
 tomoyo_check_open_permission+0x2af/0x3c0 security/tomoyo/file.c:776
 tomoyo_file_open+0x6b/0x90 security/tomoyo/tomoyo.c:334
 security_file_open+0xb5/0x1e0 security/security.c:2739
 do_dentry_open+0x588/0x14d0 fs/open.c:924
 vfs_open+0x82/0x3f0 fs/open.c:1052
 do_open fs/namei.c:4700 [inline]
 path_openat+0x2873/0x4280 fs/namei.c:4863
 do_file_open+0x20e/0x430 fs/namei.c:4892
 do_sys_openat2+0x10f/0x1e0 fs/open.c:1368
page last free pid 5160 tgid 5160 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 __free_pages_prepare mm/page_alloc.c:1406 [inline]
 __free_frozen_pages+0x79f/0x1090 mm/page_alloc.c:2950
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x47/0xf0 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x1a0/0x1f0 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:350
 kasan_slab_alloc include/linux/kasan.h:253 [inline]
 slab_post_alloc_hook mm/slub.c:4584 [inline]
 slab_alloc_node mm/slub.c:4917 [inline]
 kmem_cache_alloc_noprof+0x26b/0x6a0 mm/slub.c:4931
 alloc_filename fs/namei.c:147 [inline]
 do_getname+0x35/0x390 fs/namei.c:187
 class_filename_flags_constructor include/linux/fs.h:2566 [inline]
 do_sys_openat2+0xc7/0x1e0 fs/open.c:1367
 do_sys_open fs/open.c:1374 [inline]
 __do_sys_openat fs/open.c:1390 [inline]
 __se_sys_openat fs/open.c:1385 [inline]
 __x64_sys_openat+0x12d/0x210 fs/open.c:1385
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff88802cf7d880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88802cf7d900: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88802cf7d980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                                       ^
 ffff88802cf7da00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88802cf7da80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/07 09:38 upstream fcaeecb8b0cd c38879ea .config console log report info [disk image (non-bootable)] [vmlinux] [kernel image] ci-qemu-upstream KASAN: slab-out-of-bounds Read in cw_putcs
* Struck through repros no longer work on HEAD.