Re: [RFC v3 PATCH 5/5] x86: check VM_DEAD flag in page fault

[Laurent Dufour]

On 02/07/2018 14:45, Michal Hocko wrote:
> On Mon 02-07-18 14:26:09, Laurent Dufour wrote:
>> On 02/07/2018 14:15, Michal Hocko wrote:
>>> On Mon 02-07-18 10:45:03, Laurent Dufour wrote:
>>>> On 30/06/2018 00:39, Yang Shi wrote:
>>>>> Check VM_DEAD flag of vma in page fault handler, if it is set, trigger
>>>>> SIGSEGV.
>>>>>
>>>>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>>>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>>>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>>>>> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>>>>> ---
>>>>>  arch/x86/mm/fault.c | 4 ++++
>>>>>  1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>>>>> index 9a84a0d..3fd2da5 100644
>>>>> --- a/arch/x86/mm/fault.c
>>>>> +++ b/arch/x86/mm/fault.c
>>>>> @@ -1357,6 +1357,10 @@ static inline bool smap_violation(int error_code, struct pt_regs *regs)
>>>>>  		bad_area(regs, error_code, address);
>>>>>  		return;
>>>>>  	}
>>>>> +	if (unlikely(vma->vm_flags & VM_DEAD)) {
>>>>> +		bad_area(regs, error_code, address);
>>>>> +		return;
>>>>> +	}
>>>>
>>>> This will have to be done for all the supported architectures, what about doing
>>>> this check in handle_mm_fault() and return VM_FAULT_SIGSEGV ?
>>>
>>> We already do have a model for that. Have a look at MMF_UNSTABLE.
>>
>> MMF_UNSTABLE is a mm's flag, here this is a VMA's flag which is checked.
> 
> Yeah, and we have the VMA ready for all places where we do check the
> flag. check_stable_address_space can be made to get vma rather than mm.

Yeah, this would have been more efficient to check that flag at the beginning
of the page fault handler rather than the end, but this way it will be easier
to handle the speculative page fault too ;)