[v6.1] KASAN: slab-out-of-bounds Read in soft_cursor

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 090666d3cc90 Linux 6.1.177
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=134d0b51580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f5aae664b2df43c2
dashboard link: https://syzkaller.appspot.com/bug?extid=1c89534b655bcf1739e4
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/0b218516a5e9/disk-090666d3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3239ad568bdf/vmlinux-090666d3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a8b19cfbec3e/bzImage-090666d3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1c8953...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-out-of-bounds in soft_cursor+0x56a/0xc30 drivers/video/fbdev/core/softcursor.c:61
Read of size 1 at addr ffff888055721d3e by task syz.2.12236/30998

CPU: 1 PID: 30998 Comm: syz.2.12236 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x24e lib/dump_stack.c:106
print_address_description mm/kasan/report.c:316 [inline]
print_report+0xa8/0x210 mm/kasan/report.c:420
kasan_report+0x10b/0x140 mm/kasan/report.c:524
soft_cursor+0x56a/0xc30 drivers/video/fbdev/core/softcursor.c:61
bit_cursor+0x1699/0x1f00 drivers/video/fbdev/core/bitblit.c:404
set_cursor drivers/tty/vt/vt.c:920 [inline]
con_flush_chars+0x1f6/0x270 drivers/tty/vt/vt.c:3368
con_write+0x29/0x40 drivers/tty/vt/vt.c:3296
process_output_block drivers/tty/n_tty.c:586 [inline]
n_tty_write+0xd61/0x11e0 drivers/tty/n_tty.c:2377
do_tty_write drivers/tty/tty_io.c:1018 [inline]
file_tty_write+0x4c7/0x840 drivers/tty/tty_io.c:1089
call_write_iter include/linux/fs.h:2265 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x482/0x9a0 fs/read_write.c:584
ksys_write+0x14d/0x260 fs/read_write.c:637
do_syscall_x64 arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f43bcf9de99
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:00007f43bde0b028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f43bd225fa0 RCX: 00007f43bcf9de99
RDX: 0000000000001006 RSI: 0000200000001980 RDI: 0000000000000003
RBP: 00007f43bd033eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f43bd226038 R14: 00007f43bd225fa0 R15: 00007ffea5f4a688
</TASK>

Allocated by task 12686:
kasan_save_stack mm/kasan/common.c:46 [inline]
kasan_set_track+0x4b/0x70 mm/kasan/common.c:53
____kasan_kmalloc mm/kasan/common.c:375 [inline]
__kasan_kmalloc+0x8e/0xa0 mm/kasan/common.c:384
kasan_kmalloc include/linux/kasan.h:211 [inline]
__do_kmalloc_node mm/slab_common.c:936 [inline]
__kmalloc+0xb3/0x240 mm/slab_common.c:949
kmalloc include/linux/slab.h:568 [inline]
fbcon_set_font+0x581/0x1030 drivers/video/fbdev/core/fbcon.c:2529
con_font_set drivers/tty/vt/vt.c:4678 [inline]
con_font_op+0xac1/0xdb0 drivers/tty/vt/vt.c:4724
vt_k_ioctl drivers/tty/vt/vt_ioctl.c:474 [inline]
vt_ioctl+0x1635/0x1cc0 drivers/tty/vt/vt_ioctl.c:752
tty_ioctl+0x7e6/0xba0 drivers/tty/tty_io.c:2776
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl+0xfa/0x170 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x68/0xd2

Last potentially related work creation:
kasan_save_stack+0x3a/0x60 mm/kasan/common.c:46
__kasan_record_aux_stack+0xb2/0xc0 mm/kasan/generic.c:486
kvfree_call_rcu+0x103/0x890 kernel/rcu/tree.c:3405
neigh_periodic_work+0x3f2/0xd70 net/core/neighbour.c:1010
process_one_work+0x8ab/0x1160 kernel/workqueue.c:2292
worker_thread+0xaf5/0x12a0 kernel/workqueue.c:2439
kthread+0x29d/0x330 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

The buggy address belongs to the object at ffff888055721800
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 318 bytes to the right of
1024-byte region [ffff888055721800, ffff888055721c00)

The buggy address belongs to the physical page:
page:ffffea000155c800 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888055724800 pfn:0x55720
head:ffffea000155c800 order:3 compound_mapcount:0 compound_pincount:0
flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000010200 ffffea0000be8e08 ffffea0000769208 ffff888017441dc0
raw: ffff888055724800 0000000000100003 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0x1d20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL), pid 4271, tgid 4271 (syz-executor), ts 82507315715, free_ts 25203853131
set_page_owner include/linux/page_owner.h:31 [inline]
post_alloc_hook+0x173/0x1a0 mm/page_alloc.c:2560
prep_new_page mm/page_alloc.c:2567 [inline]
get_page_from_freelist+0x206b/0x2180 mm/page_alloc.c:4358
__alloc_pages+0x1ec/0x4f0 mm/page_alloc.c:5658
alloc_slab_page+0x5d/0x180 mm/slub.c:1799
allocate_slab mm/slub.c:1944 [inline]
new_slab+0x87/0x2d0 mm/slub.c:1997
___slab_alloc+0xbc5/0x1240 mm/slub.c:3154
__slab_alloc mm/slub.c:3240 [inline]
slab_alloc_node mm/slub.c:3325 [inline]
__kmem_cache_alloc_node+0x126/0x270 mm/slub.c:3398
__do_kmalloc_node mm/slab_common.c:935 [inline]
__kmalloc_node_track_caller+0xa0/0x240 mm/slab_common.c:956
kmalloc_reserve net/core/skbuff.c:446 [inline]
__alloc_skb+0x226/0x7d0 net/core/skbuff.c:515
alloc_skb include/linux/skbuff.h:1303 [inline]
nlmsg_new include/net/netlink.h:998 [inline]
rtmsg_fib+0xd7/0x440 net/ipv4/fib_semantics.c:522
fib_table_insert+0xd4e/0x1af0 net/ipv4/fib_trie.c:1385
fib_magic+0x2ca/0x3b0 net/ipv4/fib_frontend.c:1118
fib_add_ifaddr+0x372/0x5c0 net/ipv4/fib_frontend.c:1155
fib_netdev_event+0x361/0x470 net/ipv4/fib_frontend.c:1500
notifier_call_chain kernel/notifier.c:87 [inline]
raw_notifier_call_chain+0xcb/0x160 kernel/notifier.c:455
call_netdevice_notifiers_extack net/core/dev.c:2052 [inline]
call_netdevice_notifiers net/core/dev.c:2066 [inline]
__dev_notify_flags+0x1a7/0x310 net/core/dev.c:8715
page last free stack trace:
reset_page_owner include/linux/page_owner.h:24 [inline]
free_pages_prepare mm/page_alloc.c:1487 [inline]
free_pcp_prepare mm/page_alloc.c:1537 [inline]
free_unref_page_prepare+0x8e5/0x9e0 mm/page_alloc.c:3414
free_unref_page+0x2e/0x3f0 mm/page_alloc.c:3509
free_contig_range+0x9d/0x150 mm/page_alloc.c:9626
destroy_args+0xef/0xa0e mm/debug_vm_pgtable.c:1031
debug_vm_pgtable+0x33c/0x38e mm/debug_vm_pgtable.c:1359
do_one_initcall+0x257/0x800 init/main.c:1309
do_initcall_level+0x13d/0x1ed init/main.c:1382
do_initcalls+0x4b/0x8a init/main.c:1398
kernel_init_freeable+0x401/0x5ab init/main.c:1637
kernel_init+0x19/0x1b0 init/main.c:1525
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

Memory state around the buggy address:
ffff888055721c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888055721c80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888055721d00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff888055721d80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888055721e00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


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