Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Peter Zijlstra]

On Wed, Dec 13, 2017 at 07:14:41AM -0800, Dave Hansen wrote:
> On 12/13/2017 04:57 AM, Kirill A. Shutemov wrote:
> > Dave, what is effect of this on protection keys?
> 
> The goal was to make pkeys-protected userspace memory access
> _consistent_ with normal access.  Specifically, we want a kernel to
> disallow access (or writes) to memory where userspace mapping has a pkey
> whose permissions are in conflict with the access.
> 
> For instance:
> 
> This will fault writing a byte to 'addr':
> 
> 	char *addr = malloc(PAGE_SIZE);
> 	pkey_mprotect(addr, PAGE_SIZE, 13);
> 	pkey_deny_access(13);
> 	*addr[0] = 'f';
> 
> But this will write one byte to addr successfully (if it uses the kernel
> mapping of the physical page backing 'addr'):
> 
> 	char *addr = malloc(PAGE_SIZE);
> 	pkey_mprotect(addr, PAGE_SIZE, 13);
> 	pkey_deny_access(13);
> 	read(fd, addr, 1);
> 

This seems confused to me; why are these two cases different?