linux-kernel - Re: [RFC PATCH v1 24/28] riscv: select config for shadow stack and landing pad instr support

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Date: Thu, 25 Jan 2024 11:26:47 -0800
From: Deepak Gupta <debug@...osinc.com>
To: Conor Dooley <conor@...nel.org>
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Subject: Re: [RFC PATCH v1 24/28] riscv: select config for shadow stack and
 landing pad instr support

On Thu, Jan 25, 2024 at 06:44:48PM +0000, Conor Dooley wrote:
>On Thu, Jan 25, 2024 at 10:12:16AM -0800, Deepak Gupta wrote:
>> On Thu, Jan 25, 2024 at 06:04:26PM +0000, Conor Dooley wrote:
>> > On Wed, Jan 24, 2024 at 10:21:49PM -0800, debug@...osinc.com wrote:
>> > > From: Deepak Gupta <debug@...osinc.com>
>> > >
>> > > This patch selects config shadow stack support and landing pad instr
>> > > support. Shadow stack support and landing instr support is hidden behind
>> > > `CONFIG_RISCV_USER_CFI`. Selecting `CONFIG_RISCV_USER_CFI` wires up path
>> > > to enumerate CPU support and if cpu support exists, kernel will support
>> > > cpu assisted user mode cfi.
>> > >
>> > > Signed-off-by: Deepak Gupta <debug@...osinc.com>
>> > > ---
>> > >  arch/riscv/Kconfig | 15 +++++++++++++++
>> > >  1 file changed, 15 insertions(+)
>> > >
>> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> > > index 9d386e9edc45..437b2f9abf3e 100644
>> > > --- a/arch/riscv/Kconfig
>> > > +++ b/arch/riscv/Kconfig
>> > > @@ -163,6 +163,7 @@ config RISCV
>> > >  	select SYSCTL_EXCEPTION_TRACE
>> > >  	select THREAD_INFO_IN_TASK
>> > >  	select TRACE_IRQFLAGS_SUPPORT
>> > > +	select RISCV_USER_CFI
>> >
>> > This select makes no sense to me, it will unconditionally enable
>> > RISCV_USER_CFI. I don't think that that is your intent, since you have a
>> > detailed option below that allows the user to turn it on or off.
>> >
>> > If you remove it, the commit message will need to change too FYI.
>> >
>>
>> Selecting this config puts support in Kernel so that it can run tasks who wants
>> to enable hardware assisted control flow integrity for themselves. But apps still
>> always need to optin using `prctls`. Those prctls are stubs and return EINVAL when
>> this config is not selected. Not selecting this config means, kernel will not support
>> enabling this feature for user mode.
>
>I don't think you understand me. "select RISCV_USER_CFI" will
>unconditionally build it into the kernel, making stubs etc useless.
>You're talking like (and the rest of your commit implements it!) that
>this feature can be enabled in menuconfig etc. Having this select
>will always enable the config option, rendering the choice below
>redundant. Try turning it off in menuconfig.

Aah got it now. Thanks.
I'll fix this messaging and select in next version.

>
>Oh and if it were valid to have here, you put it in out of order. That's
>an alphanumerically sorted list :)

Thanks for pointing that out. Will fix it.
>
>Cheers,
>Conor.
>