linux-kernel - Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack support

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Date:   Thu, 29 Apr 2021 11:12:19 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack
 support

On Wed, Apr 28, 2021 at 11:39:00AM -0700, Yu, Yu-cheng wrote:
> Sorry about that.  After that email thread, we went ahead to separate shadow
> stack and ibt into different files.  I thought about the struct, the file
> names cet.h, etc.  The struct still needs to include ibt status, and if it
> is shstk_desc, the name is not entirely true.  One possible approach is, we
> don't make it a struct here, and put every item directly in thread_struct.
> However, the benefit of putting all in a struct is understandable (you might
> argue the opposite :-)).  Please make the call, and I will do the change.

/me looks forward into the patchset...

So this looks like the final version of it:

@@ -15,6 +15,7 @@ struct cet_status {
 	unsigned long	shstk_base;
 	unsigned long	shstk_size;
 	unsigned int	locked:1;
+	unsigned int	ibt_enabled:1;
 };

If so, that thing should be simply:

	struct cet {
		unsigned long shstk_base;
		unsigned long shstk_size;
		unsigned int shstk_lock : 1,
			     ibt	: 1;
	}

Is that ibt flag per thread or why is it here? I guess I'll find out.

/me greps...

ah yes, it is.

> Yes, the comments are in patch #23: Handle thread shadow stack.  I wanted to
> add that in the patch that takes the path.

That comes next, I'll look there.

> > vm_munmap() can return other negative error values, where are you
> > handling those?
> > 
> 
> For other error values, the loop stops.

And then what happens?

> > > +	cet->shstk_base = 0;
> > > +	cet->shstk_size = 0;

You clear those here without even checking whether unmap failed somehow.
And then stuff leaks but we don't care, right?

Someone else's problem, I'm sure.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette