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

[Yu, Yu-cheng]

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