linux-kernel - Re: [PATCH v19 00/27] riscv control-flow integrity for usermode

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Message-ID: <aN6sNFBzx8NPU3Th@debug.ba.rivosinc.com>
Date: Thu, 2 Oct 2025 09:45:40 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Florian Weimer <fweimer@...hat.com>
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Subject: Re: [PATCH v19 00/27] riscv control-flow integrity for usermode

On Thu, Oct 02, 2025 at 01:45:48PM +0200, Florian Weimer wrote:
>* Deepak Gupta:
>
>> On Tue, Sep 30, 2025 at 11:20:32AM +0200, Florian Weimer wrote:
>>>* Deepak Gupta:
>>>
>>>> In case of shadow stack, it similar situation. If enabled compiler
>>>> decides to insert sspush and sspopchk. They necessarily won't be
>>>> prologue or epilogue but somewhere in function body as deemed fit by
>>>> compiler, thus increasing the complexity of runtime patching.
>>>>
>>>> More so, here are wishing for kernel to do this patching for usermode
>>>> vDSO when there is no guarantee of such of rest of usermode (which if
>>>> was compiled with shadow stack would have faulted before vDSO's
>>>> sspush/sspopchk if ran on pre-zimop hardware)
>>>
>>>I think this capability is desirable so that you can use a distribution
>>>kernel during CFI userspace bringup.
>>
>> I didn't get it, can you elaborate more.
>>
>> Why having kernel carry two vDSO (one with shadow stack and one without) would
>> be required to for CFI userspace bringup?
>>
>> If Distro is compiling for RVA23 CONFIG_RISCV_USERCFI has to be selected yes,
>> kernel can have vDSO with shadow stack. Distro can light this option only when
>> its compiling entire distro for RVA23.
>
>I think it boils down to whether you want CFI bringup contributions from
>people who do not want to or cannot build their own custom RVA23
>kernels.

How will they contribute to CFI bringup without having a CFI compiled usersapce?
If their userspace is compiled with shadow stack instructions and they can't take
this userspace to old hardware else it'll start faulting as soon as control is
given to userspace (first sspush or sspopcheck in userspace).

>
>Another use case would be running container images with CFI on a
>distribution kernel which supports pre-RVA23 hardware.

Container image with CFI will have glibc and ld (and all other userspace) also
compiled with shadow stack instructions in it. As soon as you take this
container image to a pre-RVA23 hardware, you won't even reach vDSO. It'll break
much before that, unless kernel is taking a trap on all sspush/sspopchk
instructions in prologue/epilogue of functions in userspace (glibc, ld, etc)

>
>Thanks,
>Florian
>