linux-kernel - Re: [PATCH RFT v15 4/8] fork: Add shadow stack support to clone3()

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Message-ID: <e0bd1091-0342-447e-a74d-4d2ce01144fa@sirena.org.uk>
Date: Tue, 15 Apr 2025 13:56:24 +0100
From: Mark Brown <broonie@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH RFT v15 4/8] fork: Add shadow stack support to clone3()

On Mon, Apr 14, 2025 at 06:28:14PM +0000, Edgecombe, Rick P wrote:

> First of all, sorry for not contributing on this since v9. I've had an unusual
> enormous project conflict (TDX) combined with my test HW dieing.

No worries, there doesn't seem to have been huge urgency on this one :(

> Both fixed in the diff below, but in debugging the off-by-one errors I've
> realized this implementation wastes a shadow stack frame.

I rolled your diff into the series, thanks.

> Do we want this? On arm there is SHADOW_STACK_SET_MARKER, which leaves a marker
> token. But on clone3 it will also leave behind a zero frame from the CMPXCHGed
> token. So if you use SHADOW_STACK_SET_MARKER you get two marker tokens. And on
> x86 you will get one one for clone3 but not others, until x86 implements
> SHADOW_STACK_SET_MARKER. At which point x86 has to diverge from arm (bad) or
> also have the double marker frame.

> The below fixes the x86 functionally, but what do you think of the wasted frame?
> One fix would be to change shadow_stack_pointer to shadow_stack_token, and then
> have each arch consume it in the normal HW way, leaving the new thread with:

>    SSP = clone_args->shadow_stack_token + 8

I think that's a good point with the extra frame, your suggestion is
sensible.  This didn't translate well when refactoring from specifying
the extent of the shadow stack.

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