linux-kernel - Re: [syzbot] [fbdev?] KASAN: vmalloc-out-of-bounds Write in imageblit (6)

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Message-ID: <691c279e.a70a0220.3124cb.00b5.GAE@google.com>
Date: Tue, 18 Nov 2025 00:00:30 -0800
From: syzbot <syzbot+5a40432dfe8f86ee657a@...kaller.appspotmail.com>
To: deller@....de, dri-devel@...ts.freedesktop.org, 
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org, simona@...ll.ch, 
	soci@....rulez.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [fbdev?] KASAN: vmalloc-out-of-bounds Write in imageblit (6)

syzbot has found a reproducer for the following issue on:

HEAD commit:    e7c375b18160 Merge tag 'vfs-6.18-rc7.fixes' of gitolite.ke..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15476692580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1cd7f786c0f5182f
dashboard link: https://syzkaller.appspot.com/bug?extid=5a40432dfe8f86ee657a
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=10a70332580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=173228b4580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1fcb660703f1/disk-e7c375b1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/4bf314965321/vmlinux-e7c375b1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/456b373fea36/bzImage-e7c375b1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5a40432dfe8f86ee657a@...kaller.appspotmail.com

RDX: 0000000000000007 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fff34de03f0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fd0417e5fa0 R14: 00007fd0417e5fa0 R15: 0000000000000003
 </TASK>
==================================================================
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 ffffc90003749fc0 by task syz.0.17/6037

CPU: 0 UID: 0 PID: 6037 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
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
 cw_putcs_aligned drivers/video/fbdev/core/fbcon_cw.c:110 [inline]
 cw_putcs+0x917/0xbb0 drivers/video/fbdev/core/fbcon_cw.c:158
 fbcon_putcs+0x387/0x450 drivers/video/fbdev/core/fbcon.c:1320
 do_update_region+0x2e9/0x3f0 drivers/tty/vt/vt.c:628
 redraw_screen+0x63f/0x760 drivers/tty/vt/vt.c:980
 fbcon_modechanged+0x456/0x6b0 drivers/video/fbdev/core/fbcon.c:2710
 fbcon_rotate drivers/video/fbdev/core/fbcon.c:228 [inline]
 rotate_store+0x258/0x2f0 drivers/video/fbdev/core/fbcon.c:3215
 dev_attr_store+0x58/0x80 drivers/base/core.c:2437
 sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:142
 kernfs_fop_write_iter+0x3af/0x570 fs/kernfs/file.c:352
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x7d3/0x11d0 fs/read_write.c:686
 ksys_write+0x12a/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd04158f6c9
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:00007fff34de0398 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fd0417e5fa0 RCX: 00007fd04158f6c9
RDX: 0000000000000007 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fff34de03f0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fd0417e5fa0 R14: 00007fd0417e5fa0 R15: 0000000000000003
 </TASK>

The buggy address belongs to a vmalloc virtual mapping
Memory state around the buggy address:
 ffffc90003749e80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc90003749f00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>ffffc90003749f80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
                                           ^
 ffffc9000374a000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc9000374a080: 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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