Re: [PATCH] ext4: fix out-of-bounds issue in ext4_xattr_set_entry

Qianqiang Liu
2024-10-01T10:15:29Z

Hi Ojaswin,

On Tue, Oct 01, 2024 at 03:11:44PM +0530, Ojaswin Mujoo wrote:
>
> Hey Qianqiang,
>
> Thanks for the patch. I'm still reviewing this codepath but I do have
> some questions around the patch. So I understand that xattrs are
> arranged in the following format:
>
> * +------------------+
> * | header |
> * | entry 1 |
> * | entry 2 |
> * | entry 3 |
> * | four null bytes | <--- last
> * | . . . |
> * | . . . | <--- here
> * | . . . |
> * | value 1 |
> * | value 3 |
> * | value 2 |
> * +------------------+
>
> Now, in this error, my understanding is that we are actually ending up
> in a case where "here" ie the place where the new xattr entry will go is
> beyond the "last" ie the last slot for xattr entry and that is causing
> an underflow, something like the above diagram.
>
> My only concern is that why were we not able to detect this in the logic
> near the start of the function where we explcity check if we have enough
> space?
>
> Perhaps we should be fixing the logic in that if {..} instead
> since the comment a few lines above your fix:
>
> /* No failures allowed past this point. */
>
> does suggest that we can't error out below that point, so ideally all
> the checks would have been done before that.
>
> I'm still going through the issue, will update here if needed.
>
> Regards,
> ojaswin
>

Thank you for your suggestions.
I will investigate this issue further. If there are any updates, I will
let you know.

--
Best,
Qianqiang Liu