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Subject: Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble
 memory as ROX
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <88a62f834688ed77d08c778e1e427014cf7d3c1b.camel@intel.com>
Date:   Mon, 5 Jun 2023 14:11:26 -0700
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To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Precedence: bulk


> On Jun 5, 2023, at 9:10 AM, Edgecombe, Rick P =
<rick.p.edgecombe@intel.com> wrote:
>=20
> On Mon, 2023-06-05 at 11:11 +0300, Mike Rapoport wrote:
>> On Sun, Jun 04, 2023 at 10:52:44PM -0400, Steven Rostedt wrote:
>>> On Thu, 1 Jun 2023 16:54:36 -0700
>>> Nadav Amit <nadav.amit@gmail.com> wrote:
>>>=20
>>>>> The way text_poke() is used here, it is creating a new writable
>>>>> alias
>>>>> and flushing it for *each* write to the module (like for each
>>>>> write of
>>>>> an individual relocation, etc). I was just thinking it might
>>>>> warrant
>>>>> some batching or something. =20
>>=20
>>>> I am not advocating to do so, but if you want to have many
>>>> efficient
>>>> writes, perhaps you can just disable CR0.WP. Just saying that if
>>>> you
>>>> are about to write all over the memory, text_poke() does not
>>>> provide
>>>> too much security for the poking thread.
>>=20
>> Heh, this is definitely and easier hack to implement :)
>=20
> I don't know the details, but previously there was some strong dislike
> of CR0.WP toggling. And now there is also the problem of CET. Setting
> CR0.WP=3D0 will #GP if CR4.CET is 1 (as it currently is for kernel =
IBT).
> I guess you might get away with toggling them both in some controlled
> situation, but it might be a lot easier to hack up then to be made
> fully acceptable. It does sound much more efficient though.

Thanks for highlighting this issue. I understand the limitations of
CR0.WP. There is also always the concerns that without CET or other
control flow integrity mechanism, someone would abuse (using ROP/JOP)
functions that clear CR0.WP=E2=80=A6