linux-kernel - Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET

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Date:   Tue, 15 Nov 2022 22:23:47 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>,
        "Schimpe, Christina" <christina.schimpe@...el.com>
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        "keescook@...omium.org" <keescook@...omium.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
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Subject: Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET

+ Christina

On Tue, 2022-11-15 at 15:43 +0100, Peter Zijlstra wrote:
> On Fri, Nov 04, 2022 at 03:36:02PM -0700, Rick Edgecombe wrote:
> > From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> > 
> > Some applications (like GDB and CRIU) would like to tweak CET state
> > via
> > ptrace. This allows for existing functionality to continue to work
> > for
> > seized CET applications. Provide an interface based on the xsave
> > buffer
> > format of CET, but filter unneeded states to make the kernel’s job
> > easier.
> > 
> > There is already ptrace functionality for accessing xstate, but
> > this
> > does not include supervisor xfeatures. So there is not a completely
> > clear place for where to put the CET state. Adding it to the user
> > xfeatures regset would complicate that code, as it currently shares
> > logic with signals which should not have supervisor features.
> > 
> > Don’t add a general supervisor xfeature regset like the user one,
> > because it is better to maintain flexibility for other supervisor
> > xfeatures to define their own interface. For example, an xfeature
> > may
> > decide not to expose all of it’s state to userspace. A lot of enum
> > values remain to be used, so just put it in dedicated CET regset.
> > 
> > The only downside to not having a generic supervisor xfeature
> > regset,
> > is that apps need to be enlightened of any new supervisor xfeature
> > exposed this way (i.e. they can’t try to have generic save/restore
> > logic). But maybe that is a good thing, because they have to think
> > through each new xfeature instead of encountering issues when new a
> > new
> > supervisor xfeature was added.
> 
> Per this argument this should not use the CET XSAVE format and CET
> name
> at all, because that conflates the situation vs IBT. Enabling that
> might
> not want to follow this precedent.

Hmm, we definitely need to be able to set the SSP. Christina, does GDB
need anything else? I thought maybe toggling SHSTK_EN?

So it might end up looking pretty much the same, and it would just be
renamed and separated in concept.

> 
> > By adding a CET regset, it also has the effect of including the CET
> > state
> > in a core dump, which could be useful for debugging.
> > 
> > Inside the setter CET regset, filter out invalid state. Today this
> > includes states disallowed by the HW and states involving Indirect
> > Branch
> > Tracking which the kernel does not currently support for usersapce.
> > 
> > So this leaves three pieces of data that can be set, shadow stack
> > enablement, WRSS enablement and the shadow stack pointer. It is
> > worth
> > noting that this is separate than enabling shadow stack via the
> > arch_prctl()s.
> 
> Does this validate the SSP, when set, points to an actual valid SS
> page?

No, but that situation is already possible and has to be handled
anyway. Just unmap your shadow stack, and map whatever other type of
memory at the same address without doing a call or ret. Then you will
segfault at the first call or ret.