Re: [PATCH v1 1/2] mm: Implement memory-deny-write-execute as a prctl

[Topi Miettinen <toiwoton@gmail.com>]

On 10.11.2022 14.03, Catalin Marinas wrote:
> On Thu, Nov 10, 2022 at 11:27:14AM +0000, Joey Gouly wrote:
>> On Fri, Oct 28, 2022 at 11:51:00AM -0700, Kees Cook wrote:
>>> On Wed, Oct 26, 2022 at 04:04:56PM +0100, Joey Gouly wrote:
>>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>>> index 099468aee4d8..42eaf6683216 100644
>>>> --- a/mm/mmap.c
>>>> +++ b/mm/mmap.c
>>>> @@ -1409,6 +1409,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>>>>   			vm_flags |= VM_NORESERVE;
>>>>   	}
>>>>   
>>>> +	if (map_deny_write_exec(NULL, vm_flags))
>>>> +		return -EACCES;
>>>> +
>>>
>>> This seems like the wrong place to do the check -- that the vma argument
>>> is a hard-coded "NULL" is evidence that something is wrong. Shouldn't
>>> it live in mmap_region()? What happens with MAP_FIXED, when there is
>>> an underlying vma? i.e. an MAP_FIXED will, I think, bypass the intended
>>> check. For example, we had "c" above:
>>>
>>>       c)	mmap(PROT_READ);
>>> 	mprotect(PROT_READ|PROT_EXEC);		// fails
>>>
>>> But this would allow another case:
>>>
>>>       e)	addr = mmap(..., PROT_READ, ...);
>>> 	mmap(addr, ..., PROT_READ | PROT_EXEC, MAP_FIXED, ...);	// passes
>>
>> I can move the check into mmap_region() but it won't fix the MAP_FIXED
>> example that you showed here.
>>
>> mmap_region() calls do_mas_munmap(..) which will unmap overlapping regions.
>> However the `vma` for the 'old' region is not kept around, and a new vma will
>> be allocated later on "vma = vm_area_alloc(mm);", and the vm_flags are just set
>> to what is passed into mmap_region(), so map_deny_write_exec(vma, vm_flags)
>> will just be as good as passing NULL.
>>
>> It's possible to save the vm_flags from the region that is unmapped, but Catalin
>> suggested it might be better if that is part of a later extension, what do you
>> think?
> 
> I thought initially we should keep the behaviour close to what systemd
> achieves via SECCOMP while only relaxing an mprotect(PROT_EXEC) if the
> vma is already executable (i.e. check actual permission change not just
> the PROT_* flags).
> 
> We could pass the old vm_flags for that region (and maybe drop the vma
> pointer entirely, just check old and new vm_flags). But this feels like
> tightening slightly systemd's MDWE approach. If user-space doesn't get
> confused by this, I'm fine to go with it. Otherwise we can add a new
> flag later for this behaviour
> 
> I guess that's more of a question for Topi on whether point tightening
> point (e) is feasible/desirable.

I think we want 1:1 compatibility with seccomp() for the basic version, 
so MAP_FIXED shouldn't change the verdict. Later we can introduce more 
versions (perhaps even less strict, too) when it's requested by 
configuration, like MemoryDenyWriteExecute=[relaxed | strict].

-Topi