linux-kernel - Re: [PATCH v3] bcachefs: fix shift oob in alloc_lru_idx

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Message-Id: <D983ED6A-1206-4D3F-AAEB-4572BD81AB2B@gmail.com>
Date: Tue, 22 Oct 2024 13:05:30 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Hongbo Li <lihongbo22@...wei.com>
Cc: kent.overstreet@...ux.dev, mmpgouride@...il.com,
 linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+7f45fa9805c40db3f108@...kaller.appspotmail.com
Subject: Re: [PATCH v3] bcachefs: fix shift oob in alloc_lru_idx_fragmentation



> Hongbo Li <lihongbo22@...wei.com> wrote:
> 
> 
> 
>> On 2024/10/21 23:43, Jeongjun Park wrote:
>> The size of a.data_type is set abnormally large, causing shift-out-of-bounds.
>> To fix this, we need to add validation on a.data_type in
>> alloc_lru_idx_fragmentation().
>> Reported-by: syzbot+7f45fa9805c40db3f108@...kaller.appspotmail.com
>> Fixes: 260af1562ec1 ("bcachefs: Kill alloc_v4.fragmentation_lru")
>> Signed-off-by: Jeongjun Park <aha310510@...il.com>
>> ---
>>  fs/bcachefs/alloc_background.h | 3 +++
>>  1 file changed, 3 insertions(+)
>> diff --git a/fs/bcachefs/alloc_background.h b/fs/bcachefs/alloc_background.h
>> index f8e87c6721b1..163a67b97a40 100644
>> --- a/fs/bcachefs/alloc_background.h
>> +++ b/fs/bcachefs/alloc_background.h
>> @@ -168,6 +168,9 @@ static inline bool data_type_movable(enum bch_data_type type)
>>  static inline u64 alloc_lru_idx_fragmentation(struct bch_alloc_v4 a,
>>                            struct bch_dev *ca)
>>  {
>> +    if (a.data_type >= BCH_DATA_NR)
>> +        return 0;
>> +
> 
> oh, I found I have done the same thing in [1]("Re: [syzbot] [bcachefs?] UBSAN: shift-out-of-bounds in bch2_alloc_to_text"). But in my humble opinion, the validation changes also should be added. And in addition, move the condition about a.data_type into data_type_movable will be better. Just my personal opinion.:)
> 
> [1] https://www.spinics.net/lists/kernel/msg5412619.html

I still disagree with the fix to make data_type_movable() do the validation, 
but I think [1] is definitely a patch that needs to be added. 

However, [1] is far from preventing the shift oob vulnerability described 
in that syzbot report. Therefore, I think [1] should be written as a 
standalone patch and committed, rather than as a patch for that 
syzbot report.

> 
> Thanks,
> Hongbo
> 
>>      if (!data_type_movable(a.data_type) ||
>>          !bch2_bucket_sectors_fragmented(ca, a))
>>          return 0;
>> --