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 09:17:06 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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 4/29/2021 2:12 AM, Borislav Petkov wrote:
> 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.
> 

The lock applies to both shadow stack and ibt.  So maybe just "locked"?

>> 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.
> 

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.  Should it happen, it is a bug.  The kernel can probably do WARN().

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

(3) -EINTR: mmap_write_lock_killable(mm) fails.
	This should only happen to a pthread.  When a thread is existing, its 
siblings are holding mm->mmap_lock.  This is handled here.

Right now, in the kernel, only the munmap() syscall returns 
__vm_munmap() error code, otherwise the error is not checked.  Within 
the kernel and if -EINTR is not expected, this makes sense as explained 
above.

Thanks for questioning.  This piece needs to be correct.

Yu-cheng