Re: [syzbot] [fbdev?] KASAN: global-out-of-bounds Read in bit_putcs (3)

syzbot
2025-02-28T20:37:25Z

syzbot has found a reproducer for the following issue on:

HEAD commit: 017f704fbfb1 Merge branch 'for-next/core' into for-kernelci
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=12128864580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d6b7e15dc5b5e776
dashboard link: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
userspace arch: arm64
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1643dba0580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16128864580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d34fb306468f/disk-017f704f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6f1126558a26/vmlinux-017f704f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6d9d912bee27/Image-017f704f.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+793cf822d213be1a74f2@syzkaller.appspotmail.com

do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x54/0x168 arch/arm64/kernel/entry-common.c:744
el0t_64_sync_handler+0x84/0x108 arch/arm64/kernel/entry-common.c:762
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:600
==================================================================
BUG: KASAN: global-out-of-bounds in __fb_pad_aligned_buffer include/linux/fb.h:644 [inline]
BUG: KASAN: global-out-of-bounds in bit_putcs_aligned drivers/video/fbdev/core/bitblit.c:96 [inline]
BUG: KASAN: global-out-of-bounds in bit_putcs+0x9b8/0xe30 drivers/video/fbdev/core/bitblit.c:185
Read of size 1 at addr ffff80008be20810 by task syz-executor101/6448

CPU: 1 UID: 0 PID: 6448 Comm: syz-executor101 Not tainted 6.14.0-rc4-syzkaller-g017f704fbfb1 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
Call trace:
show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:466 (C)
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0xe4/0x150 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:408 [inline]
print_report+0x198/0x550 mm/kasan/report.c:521
kasan_report+0xd8/0x138 mm/kasan/report.c:634
__asan_report_load1_noabort+0x20/0x2c mm/kasan/report_generic.c:378
__fb_pad_aligned_buffer include/linux/fb.h:644 [inline]
bit_putcs_aligned drivers/video/fbdev/core/bitblit.c:96 [inline]
bit_putcs+0x9b8/0xe30 drivers/video/fbdev/core/bitblit.c:185
fbcon_putcs+0x390/0x5a0 drivers/video/fbdev/core/fbcon.c:1308
do_update_region+0x1e8/0x3d0 drivers/tty/vt/vt.c:609
update_region+0x1e0/0x478 drivers/tty/vt/vt.c:633
vcs_write+0x9e8/0x1180 drivers/tty/vt/vc_screen.c:698
do_loop_readv_writev fs/read_write.c:843 [inline]
vfs_writev+0x494/0x92c fs/read_write.c:1052
do_writev+0x178/0x304 fs/read_write.c:1096
__do_sys_writev fs/read_write.c:1164 [inline]
__se_sys_writev fs/read_write.c:1161 [inline]
__arm64_sys_writev+0x80/0x94 fs/read_write.c:1161
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x54/0x168 arch/arm64/kernel/entry-common.c:744
el0t_64_sync_handler+0x84/0x108 arch/arm64/kernel/entry-common.c:762
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:600

The buggy address belongs to the variable:
fontdata_8x16+0x1010/0x1480

The buggy address belongs to the virtual mapping at
[ffff80008b810000, ffff80008f6b0000) created by:
declare_kernel_vmas+0x58/0xb8 arch/arm64/mm/mmu.c:771

The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1a1820
flags: 0x5ffc00000002000(reserved|node=0|zone=2|lastcpupid=0x7ff)
raw: 05ffc00000002000 fffffdffc5860808 fffffdffc5860808 0000000000000000
raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff80008be20700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff80008be20780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff80008be20800: 00 00 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
^
ffff80008be20880: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
ffff80008be20900: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
==================================================================

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