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

[Dave Hansen]

On 12/13/2017 07:32 AM, Peter Zijlstra wrote:
>> 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?

Protection keys doesn't work in the kernel direct map, so if the read()
was implemented by writing to the direct map alias of 'addr' then this
would bypass protection keys.