linux-kernel - Re: [PATCH RFT v9 4/8] fork: Add shadow stack support to clone3()

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Message-ID: <Zv20luC6us-LEMqN@finisterre.sirena.org.uk>
Date: Wed, 2 Oct 2024 22:01:10 +0100
From: Mark Brown <broonie@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH RFT v9 4/8] fork: Add shadow stack support to clone3()

On Wed, Oct 02, 2024 at 02:42:58PM +0100, Mark Brown wrote:
> On Tue, Oct 01, 2024 at 11:03:10PM +0000, Edgecombe, Rick P wrote:

> > I'm not so sure. The thing is a regular stack can be re-used in full - just set
> > the RSP to the end and take advantage of the whole stack. A shadow stack can
> > only be used where there is a token.

> Yeah, I'm not sure how appealing it is trying to use a memory pool with
> of shadow stacks - like you say you can't reset the top of the stack so
> you need to keep track of that when the stack becomes unused.  If the
> users don't leave the SSP at the top of the stack then unless writes
> have been enabled (which has security issues) then gradually the size of
> the shadow stacks will be eroded which will need to be managed.  You
> could do it, but it's clearly not really how things are supposed to
> work.  The use case with starting a new worker thread for an existing in
> use state seems much more appealing.

BTW it's probably also worth noting that at least on arm64 (perhaps x86
is different here?) the shadow stack of a thread that exited won't have
a token placed on it so it won't be possible to use it with clone3() at
all unless another token is written.  To get a shadow stack you could
use with clone3() you'd either need to allocate a new one, pivot away
from one that's currently in use or enable shadow stack writes and place
a token.

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