linux-kernel - Re: [PATCH RFC] ubifs: Fix use-after-free in ubifs_tnc_end

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Message-ID: <239af2ee-c18d-414f-099f-2c82f98d9671@huawei.com>
Date: Wed, 16 Oct 2024 10:11:46 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: Waqar Hameed <waqar.hameed@...s.com>
CC: Richard Weinberger <richard@....at>, Sascha Hauer
	<s.hauer@...gutronix.de>, <kernel@...s.com>, <linux-mtd@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] ubifs: Fix use-after-free in ubifs_tnc_end_commit

在 2024/10/16 2:52, Waqar Hameed 写道:
Hi Waqar,
[...]
> 
> For completeness, here are the _exact_ steps that I have used to
> reproduce this on my system with v6.12-rc2 (commit 75b607fab38d "Merge
> tag 'sched_ext-for-6.12-rc2-fixes' of
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext"):
> 
> ```
> ubiattach -m 2
> 
> keyctl add logon dummy_key: dummy_load @us
> 
> ubimkvol /dev/ubi0 -s 80MiB -n 0 -N test-vol
> ubiupdatevol /dev/ubi0_0 -t
> 
> mount -t ubifs /dev/ubi0_0 /mnt/flash -o auth_hash_name=sha256,auth_key=dummy_key:
> 
> count=0
> while true; do
>      date
>      count=$(($count + 1))
>      echo count=$count
> 
>      rm -f /mnt/flash/test-file.bin
>      dd if=/dev/urandom of=/mnt/flash/test-file.bin bs=1M count=60 conv=fsync
> 
>      echo ""
> done
> ```
> 

Thanks for the complete program, I will try it again.
BTW, what is the configuration of your flash?(eg. erase size, page size)?

> Note that you need to have `CONFIG_UBIFS_FS_AUTHENTICATION=y` (and
> `CONFIG_KASAN=y` obviously) in your `.config` in order to trigger the
> offending `memcpy()` in `ubifs_copy_hash()`. Also, it takes a while. For
> example, last time it took me 88 iterations of the above loop before it
> triggered. So you might need to let it spin for a while.

Yes, above two configs are enabled, and I have added printing messages 
to confirm the authentication path is active.
> 
>>
>> Can you dig more deeper by adding more debug message, so that we can figure out
>> what is really happening.
> 
> Certainly! I could try to enable the debug prints from UBIFS, however
> they are *a lot*. Moreover, printing that much changes the timing
> behavior and might make it harder to trigger the use-after-free. Do you
> have any tips on where we should try to focus the debug prints (a
> dynamic debug filter).
> 

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?