linux-kernel - Re: [PATCH RFC RFT v2 2/5] fork: Add shadow stack support to clone3()

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Message-ID: <d90884a0-c4d3-41e9-8f23-68aa87bbe269@sirena.org.uk>
Date:   Wed, 15 Nov 2023 18:43:47 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Szabolcs.Nagy@....com" <Szabolcs.Nagy@....com>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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        "Pandey, Sunil K" <sunil.k.pandey@...el.com>
Subject: Re: [PATCH RFC RFT v2 2/5] fork: Add shadow stack support to clone3()

On Wed, Nov 15, 2023 at 04:20:12PM +0000, Szabolcs.Nagy@....com wrote:
> The 11/15/2023 12:36, Mark Brown wrote:
> > On Wed, Nov 15, 2023 at 12:45:45AM +0000, Edgecombe, Rick P wrote:
> > > On Tue, 2023-11-14 at 20:05 +0000, Mark Brown wrote:

> > > > +               if (size < 8)
> > > > +                       return (unsigned long)ERR_PTR(-EINVAL);

> > > What is the intention here? The check in map_shadow_stack is to leave
> > > space for the token, but here there is no token.

> > It was to ensure that there is sufficient space for at least one entry
> > on the stack.

> end marker token (0) needs it i guess.

x86 doesn't currently have end markers.  Actually, that's a point -
should we add a flag for specifying the use of end markers here?
There's code in my map_shadow_stack() implementation for arm64 which
does that.

> otherwise 0 size would be fine: the child may not execute
> a call instruction at all.

Well, a size of specifically zero will result in a fallback to implicit
allocation/sizing of the stack as things stand so this is specifically
the case where a size has been specified but is smaller than a single
entry.

> > > I think for CLONE_VM we should not require a non-zero size. Speaking of
> > > CLONE_VM we should probably be clear on what the expected behavior is
> > > for situations when a new shadow stack is not usually allocated.
> > > !CLONE_VM || CLONE_VFORK will use the existing shadow stack. Should we
> > > require shadow_stack_size be zero in this case, or just ignore it? I'd
> > > lean towards requiring it to be zero so userspace doesn't pass garbage
> > > in that we have to accommodate later. What we could possibly need to do
> > > around that though, I'm not sure. What do you think?

> > Yes, requiring it to be zero in that case makes sense I think.

> i think the condition is "no specified separate stack for
> the child (stack==0 || stack==sp)".

> CLONE_VFORK does not imply that the existing stack will be
> used (a stack for the child can be specified, i think both
> glibc and musl do this in posix_spawn).

That also works as a check I think, though it requires the arch to check
for the stack==sp case - I hadn't been aware of the posix_spawn() usage,
the above checks Rick suggested just follow the handling for implicit
allocation we have currently.

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