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 18:45:49 +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 Thu, Apr 29, 2021 at 09:17:06AM -0700, Yu, Yu-cheng wrote:
> The lock applies to both shadow stack and ibt.  So maybe just "locked"?

Sure.

> vm_munmap() returns error as the following:
> 
> (1) -EINVAL: address/size/alignment is wrong.
> 	For shadow stack, the kernel keeps track of it, this cannot/should not
> happen.

You mean nothing might corrupt

        cet->shstk_base
        cet->shstk_size

?

I can't count the ways I've heard "should not happen" before and then it
happening anyway.

So probably not but we better catch stuff like that instead of leaking.

> Should it happen, it is a bug.

Ack.

> The kernel can probably do WARN().

Most definitely WARN. You need to catch funsies like that. But WARN_ONCE
should be enough for now.

> (2) -ENOMEM: when doing __split_vma()/__vma_adjust(), kmem_cache_alloc()
> fails.
> 	Not much we can do.  Perhaps WARN()?

You got it.

Bottom line is: if you can check for this and it is cheap, then
definitely. Code changes, gets rewritten, reorganized, the old
assertions change significance, and so on...

Thx.

-- 
Regards/Gruss,
    Boris.

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