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

[Zhihao Cheng]

在 2024/11/7 0:36, Waqar Hameed 写道:
> Sorry for the late response Zhihao! I've been quite busy these days...
> 
> On Fri, Oct 18, 2024 at 09:40 +0800 Zhihao Cheng <chengzhihao1 at huawei.com> wrote:
> 
>> 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.
> 
> Did you manage to reproduce the issue with this?

I tried, but I still cannot reproduce it on my local machine.
> 
>>>
>>>> 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.
> 
> I'm a little worried that traversing the whole TNC could change the
> timing behavior, and thus might not trigger the race. Let's do that in
> steps? Start with the other asserts (see diff below) and later just do
> this assert. Does that sound reasonable?

Fine. I add one comment below.
> 
> I could modify `dbg_check_tnc()` so that it also checks that each dirty
> `znode` is present in `c->cnext` list. We then call this at the end of
> `get_znodes_to_commit()`.
> 

Sounds good to me, please remove other non-related checks in 
dbg_check_tnc().
>>>> 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.
> 
> [...]
> 
> To summarize, I'll start a run with the following asserts:
> 
> diff --git a/fs/ubifs/tnc_commit.c b/fs/ubifs/tnc_commit.c
> index a55e04822d16..4eef82e02afe 100644
> --- a/fs/ubifs/tnc_commit.c
> +++ b/fs/ubifs/tnc_commit.c
> @@ -652,11 +652,17 @@ static int get_znodes_to_commit(struct ubifs_info *c)
>   	}
>   	cnt += 1;
>   	while (1) {

Please move the check after the assignment of 'znode->cparent', because 
'znode->parent' could be switched by tnc_insert().
> +		ubifs_assert(c, znode->ciip < c->fantout);
> +		if (znode->cparent) {
> +			ubifs_assert(c, ubifs_zn_dirty(znode->cparent));
> +		}
> +
>   		ubifs_assert(c, !ubifs_zn_cow(znode));
>   		__set_bit(COW_ZNODE, &znode->flags);
>   		znode->alt = 0;
>   		cnext = find_next_dirty(znode);
>   		if (!cnext) {
> +			ubifs_assert(c, znode == c->zroot.znode);
>   			znode->cnext = c->cnext;
>   			break;
>   		}
> 

@@ -662,6 +662,10 @@ static int get_znodes_to_commit(struct ubifs_info *c)
                 }
                 znode->cparent = znode->parent;
                 znode->ciip = znode->iip;
+               if (znode->cparent) {
+                       ubifs_assert(c, ubifs_zn_dirty(znode->cparent));
+               }
+               ubifs_assert(c, znode->ciip < c->fantout);
                 znode->cnext = cnext;
                 znode = cnext;
                 cnt += 1;

> Then later, another run with a modified `dbg_check_tnc()` to check that
> all dirty `znode`s are indeed present in the list `c->cnext`.
> .
>