Re: [PATCH v2] ubifs: Fix use-after-free in ubifs_tnc_end_commit

[Waqar Hameed <waqar.hameed@axis.com>]

On Sat, Nov 09, 2024 at 10:34 +0800 Zhihao Cheng <chengzhihao1@huawei.com> wrote:

> 在 2024/10/9 22:46, Waqar Hameed 写道:
> 3 nits below.
>
> 1. Make the title as 'ubifs: authentication: Fix use-after-free in
> ubifs_tnc_end_commit'
>
> 2. At the begining of the commit msg, describe the problem:
> After TNC tree inserting(which may trigger a znode split and change the
> znode->parent) and deleting(which may trigger znode freeing), the
> znode->cparent(which still points to a freed znode) may not be updated at the
> begining of commit, which could trigger an UAF problem while accessing
> znode->cparent in write_index().

Alright, will rephrase the commit message a bit.

>
>> Running
>>    rm -f /etc/test-file.bin
>>    dd if=/dev/urandom of=/etc/test-file.bin bs=1M count=60 conv=fsync
>> in a loop, with `CONFIG_UBIFS_FS_AUTHENTICATION`, KASAN reports:
>>    BUG: KASAN: use-after-free in ubifs_tnc_end_commit+0xa5c/0x1950
>>    Write of size 32 at addr ffffff800a3af86c by task ubifs_bgt0_20/153
>>    Call trace:
>
> [...]
>> diff --git a/fs/ubifs/tnc_commit.c b/fs/ubifs/tnc_commit.c
>> index a55e04822d16..a464eb557585 100644
>> --- a/fs/ubifs/tnc_commit.c
>> +++ b/fs/ubifs/tnc_commit.c
>> @@ -657,6 +657,7 @@ static int get_znodes_to_commit(struct ubifs_info *c)
>>   		znode->alt = 0;
>>   		cnext = find_next_dirty(znode);
>>   		if (!cnext) {
>
> 3. I'd like to add the the assertion 'ubifs_assert(c, !znode->parent);'

Wouldn't the assert always be true? Since the root node wouldn't have a
parent. Or are we afraid of some other edge cases (bugs?) that might
have been missed and want to be defensive here? Either way, I'll add the
assert.

>> +			znode->cparent = NULL;
>>   			znode->cnext = c->cnext;
>>   			break;
>>   		}
>>