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

Zhihao Cheng
2024-11-07T07:15:16Z

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@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@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`.
> .