linux-kernel - Re: [syzbot] [fs?] UBSAN: shift-out-of-bounds in befs_check

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Message-ID: <68d99dd6.050a0220.1696c6.000a.GAE@google.com>
Date: Sun, 28 Sep 2025 13:43:02 -0700
From: syzbot <syzbot+fc26c366038b54261e53@...kaller.appspotmail.com>
To: linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	xandfury@...il.com
Subject: Re: [syzbot] [fs?] UBSAN: shift-out-of-bounds in befs_check_sb

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
UBSAN: shift-out-of-bounds in befs_check_sb

loop0: detected capacity change from 0 to 128
------------[ cut here ]------------
UBSAN: shift-out-of-bounds in fs/befs/super.c:96:9
shift exponent 2066842816 is too large for 32-bit type 'int'
CPU: 0 UID: 0 PID: 6637 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 ubsan_epilogue+0xa/0x40 lib/ubsan.c:233
 __ubsan_handle_shift_out_of_bounds+0x386/0x410 lib/ubsan.c:494
 befs_check_sb+0x421/0x4c0 fs/befs/super.c:96
 befs_fill_super+0x49b/0xa70 fs/befs/linuxvfs.c:841
 get_tree_bdev_flags+0x40e/0x4d0 fs/super.c:1692
 vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
 do_new_mount+0x2a2/0x9e0 fs/namespace.c:3808
 do_mount fs/namespace.c:4136 [inline]
 __do_sys_mount fs/namespace.c:4347 [inline]
 __se_sys_mount+0x317/0x410 fs/namespace.c:4324
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc8d5fb066a
Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 de 1a 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 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:00007fc8d5615e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007fc8d5615ef0 RCX: 00007fc8d5fb066a
RDX: 0000200000000040 RSI: 0000200000000000 RDI: 00007fc8d5615eb0
RBP: 0000200000000040 R08: 00007fc8d5615ef0 R09: 0000000000800001
R10: 0000000000800001 R11: 0000000000000246 R12: 0000200000000000
R13: 00007fc8d5615eb0 R14: 0000000000009e17 R15: 0000200000000080
 </TASK>
---[ end trace ]---


Tested on:

commit:         8f973663 Merge tag 'trace-v6.17-rc7' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17f7f142580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f5b21423ca3f0a96
dashboard link: https://syzkaller.appspot.com/bug?extid=fc26c366038b54261e53
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Note: no patches were applied.