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

syzbot
2025-09-08T19:19:28Z

Hello,

syzbot found the following issue on:

HEAD commit: 76eeb9b8de98 Linux 6.17-rc5
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14379562580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e0bea6c0b97a2002
dashboard link: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b2c9b90de8ba/disk-76eeb9b8.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/dd1bcf1fd25e/vmlinux-76eeb9b8.xz
kernel image: https://storage.googleapis.com/syzbot-assets/dc7e94f5dffb/bzImage-76eeb9b8.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 ffffc90003649000 by task syz.1.259/7231

CPU: 0 UID: 0 PID: 7231 Comm: syz.1.259 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
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
update_region+0xc1/0x160 drivers/tty/vt/vt.c:641
vcs_write+0x7c7/0xdb0 drivers/tty/vt/vc_screen.c:698
vfs_write+0x29d/0x11d0 fs/read_write.c:684
ksys_write+0x12a/0x250 fs/read_write.c:738
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7effcbf8ebe9
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:00007effccda6038 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007effcc1c6270 RCX: 00007effcbf8ebe9
RDX: 000000000000ffd8 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007effcc011e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007effcc1c6308 R14: 00007effcc1c6270 R15: 00007ffd4fcb0c08
</TASK>

The buggy address belongs to a 0-page vmalloc region starting at 0xffffc90003349000 allocated at drm_gem_shmem_vmap_locked+0x561/0x7e0 drivers/gpu/drm/drm_gem_shmem_helper.c:371
Memory state around the buggy address:
ffffc90003648f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffffc90003648f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffc90003649000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
^
ffffc90003649080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
ffffc90003649100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
==================================================================

---
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