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Re: [dm-devel] FW: RE:(2) FW: RE: FW: (2) FW: Use-after-free while dm-verity



Hi,

On Mon, Aug 20, 2018 at 2:33 PM, 정혜연 <hyeon chung samsung com> wrote:
>
> Dear Gilad,
>
>
>
> We found memcopy is not preempted as you explained.
>
> Memcopy make kernel panic at the very first time.
>
> Your comment was very helpful us to find this. Thank you.
>
>


I'm glad I could help.

>
> We found the other core executed shrink_node at that time, so we suspect that lowmem situation coulde make this problem.
>
> And we tested after reverting async patch for 3 days and there's no isuue so far.
>
> So we assume there's unexpected free in dm-verity or some page memory operation (ex. offset ?).
>
>
>
> Do you have any idea?
>
>


One thing that comes to mind is understanding why there are data
errors in the data being read. The stack trace you sent shows calls to
verity_fec_decode().
This would happen if there was a mismatch between the computed hash
and the expected hash of (in this specific case per the stack trace) a
storage block containing hashes for the data. It is therefore attempts
to fix the errors via employing Read Solomon codes.

While it is part of DM-Verity design, I'm surprised to see this happen
in a lab environment. Do you deliberately create 1 bit errors in the
data as a test or is this some coincidence?

Other than that, I return to my previous request:

"If this doesn't bring anything to light, please provide a the full
kernel panic log (not just the snippet here), your kernel config and
what architecture/board you are running this on, as well as details
about DM-verity partition setup, size and underlying storage so I may
attempt to recreate this."


This will best help me help you.


Cheers,

Gilad


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