Re: [syzbot] [fbdev?] KASAN: vmalloc-out-of-bounds Write in imageblit (5)

syzbot
2025-09-15T15:59:31Z

syzbot has found a reproducer for the following issue on:

HEAD commit: f83ec76bf285 Linux 6.17-rc6
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13e5f934580000
kernel config: https://syzkaller.appspot.com/x/.config?x=8f01d8629880e620
dashboard link: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190
compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14097b62580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17a9bb12580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-f83ec76b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bdedf70f8797/vmlinux-f83ec76b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5bf9318d9242/bzImage-f83ec76b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+48b0652a95834717f190@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: vmalloc-out-of-bounds in fb_write_offset drivers/video/fbdev/core/sysmem.h:30 [inline]
BUG: KASAN: vmalloc-out-of-bounds in fb_bitmap_2ppw drivers/video/fbdev/core/fb_imageblit.h:364 [inline]
BUG: KASAN: vmalloc-out-of-bounds in fb_bitmap_imageblit drivers/video/fbdev/core/fb_imageblit.h:462 [inline]
BUG: KASAN: vmalloc-out-of-bounds in fb_imageblit drivers/video/fbdev/core/fb_imageblit.h:492 [inline]
BUG: KASAN: vmalloc-out-of-bounds in sys_imageblit+0x1a6f/0x1e60 drivers/video/fbdev/core/sysimgblt.c:24
Write of size 8 at addr ffffc900051b1000 by task syz.0.17/6126

CPU: 2 UID: 0 PID: 6126 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xcd/0x630 mm/kasan/report.c:482
kasan_report+0xe0/0x110 mm/kasan/report.c:595
fb_write_offset drivers/video/fbdev/core/sysmem.h:30 [inline]
fb_bitmap_2ppw drivers/video/fbdev/core/fb_imageblit.h:364 [inline]
fb_bitmap_imageblit drivers/video/fbdev/core/fb_imageblit.h:462 [inline]
fb_imageblit drivers/video/fbdev/core/fb_imageblit.h:492 [inline]
sys_imageblit+0x1a6f/0x1e60 drivers/video/fbdev/core/sysimgblt.c:24
drm_fbdev_shmem_defio_imageblit+0x20/0x130 drivers/gpu/drm/drm_fbdev_shmem.c:38
bit_putcs_unaligned drivers/video/fbdev/core/bitblit.c:138 [inline]
bit_putcs+0x90f/0xde0 drivers/video/fbdev/core/bitblit.c:187
fbcon_putcs+0x384/0x4a0 drivers/video/fbdev/core/fbcon.c:1327
do_update_region+0x2e6/0x3f0 drivers/tty/vt/vt.c:627
invert_screen+0x1e4/0x590 drivers/tty/vt/vt.c:748
highlight drivers/tty/vt/selection.c:57 [inline]
clear_selection drivers/tty/vt/selection.c:87 [inline]
clear_selection+0x59/0x70 drivers/tty/vt/selection.c:83
vc_do_resize+0xd9b/0x10e0 drivers/tty/vt/vt.c:1195
vc_resize include/linux/vt_kern.h:49 [inline]
fbcon_set_disp+0x7ad/0xe50 drivers/video/fbdev/core/fbcon.c:1430
con2fb_init_display drivers/video/fbdev/core/fbcon.c:828 [inline]
set_con2fb_map+0x703/0x1080 drivers/video/fbdev/core/fbcon.c:902
fbcon_set_con2fb_map_ioctl+0x16c/0x220 drivers/video/fbdev/core/fbcon.c:3132
do_fb_ioctl+0x328/0x7e0 drivers/video/fbdev/core/fb_chrdev.c:138
fb_ioctl+0xe5/0x150 drivers/video/fbdev/core/fb_chrdev.c:169
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:598 [inline]
__se_sys_ioctl fs/ioctl.c:584 [inline]
__x64_sys_ioctl+0x18b/0x210 fs/ioctl.c:584
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8e73d8eba9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc51d189d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f8e73fd5fa0 RCX: 00007f8e73d8eba9
RDX: 0000200000000180 RSI: 0000000000004610 RDI: 0000000000000004
RBP: 00007f8e73e11e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8e73fd5fa0 R14: 00007f8e73fd5fa0 R15: 0000000000000003
</TASK>

The buggy address belongs to a 0-page vmalloc region starting at 0xffffc90004eb1000 allocated at drm_gem_shmem_vmap_locked+0x561/0x7e0 drivers/gpu/drm/drm_gem_shmem_helper.c:371
Memory state around the buggy address:
ffffc900051b0f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffffc900051b0f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffc900051b1000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
^
ffffc900051b1080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
ffffc900051b1100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
==================================================================

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#syz test: git://repo/address.git branch-or-commit-hash
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