[PATCH RFC] ubifs: Fix use-after-free in ubifs_tnc_end_commit

[Zhihao Cheng]

在 2024/10/18 2:36, Waqar Hameed 写道:
> On Wed, Oct 16, 2024 at 10:11 +0800 Zhihao Cheng <chengzhihao1 at huawei.com> wrote:
> 
> [...]
> 
>> BTW, what is the configuration of your flash?(eg. erase size, page size)?
> 
> $ mtdinfo /dev/mtd2
>    mtd2
>    Name:                           firmware
>    Type:                           nand
>    Eraseblock size:                131072 bytes, 128.0 KiB
>    Amount of eraseblocks:          1832 (240123904 bytes, 229.0 MiB)
>    Minimum input/output unit size: 2048 bytes
>    Sub-page size:                  2048 bytes
>    OOB size:                       64 bytes
>    Character device major/minor:   90:4
>    Bad blocks are allowed:         true
>    Device is writable:             true
> 
> $ ubinfo /dev/ubi0_0
>    Volume ID:   0 (on ubi0)
>    Type:        dynamic
>    Alignment:   1
>    Size:        661 LEBs (83931136 bytes, 80.0 MiB)
>    State:       OK
>    Name:        test-vol
>    Character device major/minor: 244:1
> 
> [...]

Thanks, I will change my nandsim configurations to generate a mtd device 
the same model.
> 
>> Well, let's do a preliminary analysis.
>> The znode->cparent[znode->ciip] is a freed address in write_index(), which
>> means:
>> 1. 'znode->ciip' is valid, znode->cparent is freed by tnc_delete, however znode
>> cannot be freed if znode->cnext is not NULL, which means:
>>    a) 'znode->cparent' is not dirty, we should add an assertion like
>>    ubifs_assert(c, ubifs_zn_dirty(znode->cparent)) in get_znodes_to_commit().
>>    Note, please check that 'znode->cparent' is not NULL before the assertion.
>>    b) 'znode->cparent' is dirty, but it is not added into list 'c->cnext', we
>>    should traverse the entire TNC in get_znodes_to_commit() to make sure that all
>>    dirty znodes are collected into list 'c->cnext', so another assertion is
>>   needed.
>> 2. 'znode->ciip' is invalid, and the value beyonds the memory area of
>> znode->cparent. All znodes are allocated with size of 'c->max_znode_sz', which
>> means that 'znode->ciip' exceeds the 'c->fantout', so we can add an assertion
>> like ubifs_assert(c, znode->ciip < c->fantout) in get_znodes_to_commit().
>>
>> That's what I can think of, are there any other possibilities?
> 
> I looked a little more at `get_znodes_to_commit()` when adding the
> asserts you suggest, and I have a question: what happens when
> `find_next_dirty()` returns `NULL`? In that case
> 
> ```
> znode->cnext = c->cnext;
> ```
> 
> but `znode->cparent` and `znode->ciip` are not updated. Shouldn't they?

Good thinking.
According to the implementation of find_next_dirty(), the order of dirty 
znodes collection is bottom-up, which means that the last dirty znode is 
the root znode, so it doesn't have a parent. You can verify that by 
adding assertion to check whether the last dirty znode is the root.
> 
> By the way, I left a test running, and it actually triggered the same
> KASAN report after 800 iterations... So we now at least know that this
> patch doesn't indeed fix the problem.
> 
> I also found another minor thing regarding the update of `cnt` in
> `get_znodes_to_commit`. I'll send a separate patch for that.
> .
>