linux-kernel - "divide error in bdi_set_min_bytes" in Linux kernel version 6.13.0-rc2

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Message-ID: <CAKHoSAsMYWOYfqw6h74cEzucg1vGZaY4ShT3e35NnX2v_Ro04w@mail.gmail.com>
Date: Fri, 3 Jan 2025 15:25:01 +0800
From: cheung wall <zzqq0103.hey@...il.com>
To: "Matthew Wilcox (Oracle)" <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: "divide error in bdi_set_min_bytes" in Linux kernel version 6.13.0-rc2

Hello,

I am writing to report a potential vulnerability identified in the
Linux Kernel version 6.13.0-rc2. This issue was discovered using our
custom vulnerability discovery tool.

HEAD commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4 (tag: v6.13-rc2)

Affected File: mm/page-writeback.c

File: mm/page-writeback.c

Function: bdi_set_min_bytes

Detailed Call Stack:

------------[ cut here begin]------------

RIP: 0010:div64_u64 include/linux/math64.h:69 [inline]
RIP: 0010:bdi_ratio_from_pages mm/page-writeback.c:695 [inline]
RIP: 0010:bdi_set_min_bytes+0x9f/0x1d0 mm/page-writeback.c:799
Code: ff 48 39 d8 0f 82 3b 01 00 00 e8 ac fd e7 ff 48 69 db 40 42 0f
00 48 8d 74 24 40 48 8d 7c 24 20 e8 c6 f1 ff ff 31 d2 48 89 d8 <48> f7
74 24 40 48 89 c3 3d 40 42 0f 00 0f 87 08 01 00 00 e8 79 fd
RSP: 0018:ffff88810a5f7b60 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff9c9ef057
RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff88810a5f7ab8
RBP: 1ffff110214bef6c R08: 0000000000000000 R09: fffffbfff4081c7b
R10: ffffffffa040e3df R11: 0000000000032001 R12: ffff888105c65000
R13: dffffc0000000000 R14: ffff888105c65000 R15: ffff888105c65800
FS: 00007fdfc7c37580(0000) GS:ffff88811b280000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055adcdc786c8 CR3: 0000000104128000 CR4: 0000000000350ef0
Call Trace:
<TASK>
min_bytes_store+0xba/0x120 mm/backing-dev.c:385
dev_attr_store+0x58/0x80 drivers/base/core.c:2439
sysfs_kf_write+0x136/0x1a0 fs/sysfs/file.c:139
kernfs_fop_write_iter+0x323/0x530 fs/kernfs/file.c:334
new_sync_write fs/read_write.c:586 [inline]
vfs_write+0x51e/0xc80 fs/read_write.c:679
ksys_write+0x110/0x200 fs/read_write.c:731
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xa6/0x1a0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fdfc7b4d513
Code: 8b 15 81 29 0e 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f
1f 00 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 01 00 00 00 0f 05 <48> 3d
00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 48 89 54 24 18
RSP: 002b:00007ffe7796ae28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000055adcdc766c0 RCX: 00007fdfc7b4d513
RDX: 0000000000000002 RSI: 000055adcdc766c0 RDI: 0000000000000001
RBP: 0000000000000002 R08: 000055adcdc766c0 R09: 00007fdfc7c30be0
R10: 0000000000000070 R11: 0000000000000246 R12: 0000000000000001
R13: 0000000000000002 R14: 7fffffffffffffff R15: 0000000000000000

------------[ cut here end]------------

Root Cause:

The crash is caused by a division by zero error within the Linux
kernel's page-writeback subsystem. Specifically, the bdi_set_min_bytes
function attempts to calculate a ratio using bdi_ratio_from_pages,
which internally calls div64_u64. During this calculation, a
denominator value unexpectedly becomes zero, likely due to improper
handling or validation of input data provided through the sysfs
interface during the min_bytes_store operation. This erroneous zero
value leads to a divide error exception when the kernel tries to
perform the division. The issue occurs while processing a sysfs write
operation (min_bytes_store), suggesting that invalid or uninitialized
data supplied through sysfs triggers the faulty calculation,
ultimately causing the kernel to crash.

Thank you for your time and attention.

Best regards

Wall