Re: [uml-devel] Kernel access to user memory in SKAS

[Jeff Dike <jdike@addtoit.com>]

On Fri, Feb 23, 2007 at 01:36:12PM -0500, Bryan Parno wrote:
> 	I'm interested in mediating accesses by the UML guest kernel to  
> memory used by the UML guest processes.  At present, I'm looking at a  
> scenario using SKAS3, so the kernel has an address space distinct  
> from that of the user processes.  I guess I'm a bit confused as to  
> how the guest kernel actually manages to touch guest process memory  
> (e.g., for copying system call arguments).  Looking through /arch/um/ 
> kernel/skas/uaccess.c, it appears that all of these accesses devolve  
> to performing a strncpy or memcpy, but I don't see a translation from  
> an address in the user's address space to one in the kernel's address  
> space.  Does this happen somewhere else, or am I misunderstanding  
> SKAS? 

So far, you're fine.  What you're missing is, i.e.:

int copy_from_user_skas(void *to, const void __user *from, int n)
{
	if(segment_eq(get_fs(), KERNEL_DS)){
		memcpy(to, (__force void*)from, n);
		return(0);
	}

	return(access_ok(VERIFY_READ, from, n) ?
	       buffer_op((unsigned long) from, n, 0, copy_chunk_from_user, &to):
	       n);
}

The buffer_op case handles userspace memory.  It does the following:
	figures out where in kernel physical memory the userspace data is
	breaks the operation across pages if necessary
	wraps enough state around the operation to be able to handle
page faults and finish it after the page has been faulted in

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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