Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

[David Hildenbrand <david@redhat.com>]

>>
>> Well, there is long-term pinning that can break COW and other weird stuff
>> like FOLL_FORCE. Most of the latter probably holds the mmap lock in write
>> mode. Probably.
> 
> Well GUP uses read lock.

Right, so it can race with MADV_DONTNEED.

> 
> FOLL_FORCE won't override anything as we have this check in check_vma_flags():
> 
> 	if (write) {
> 		if (!vma_anon &&
> 		    !writable_file_mapping_allowed(vma, gup_flags))
> 			return -EFAULT;
> 
> 		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
> 			if (!(gup_flags & FOLL_FORCE))
> 				return -EFAULT;
> 			/*
> 			 * We used to let the write,force case do COW in a
> 			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
> 			 * set a breakpoint in a read-only mapping of an
> 			 * executable, without corrupting the file (yet only
> 			 * when that file had been opened for writing!).
> 			 * Anon pages in shared mappings are surprising: now
> 			 * just reject it.
> 			 */
> 			if (!is_cow_mapping(vm_flags))
> 				return -EFAULT;
> 		}
> 	}
> 
> With:
> 
> static inline bool is_cow_mapping(vm_flags_t flags)
> {
> 	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
> }
> 

Not sure what you mean. Using FOLL_FORCE you can write into MAP_PRIVATE 
R/O mappings. Particular useful for installing breakpoints into loaded 
executables etc.

is_cow_mapping() tells you exactly that: the only place where we can 
have anon folios is when we have a MAP_PRIVATE mapping (!VM_SHARED) that 
can be writable, for example, through mprotect(PROT_WRITE) (VM_MAYWRITE).

A MAP_PRIVATE R/O file mapping matches is_cow_mapping().

> So - we explicitly disallow FOLL_FORCE write override for CoW file-backed
> mappings.
> 
> Obviously if FOLL_FORCE is not set, then we're ALSO not allowed to get past a
> FOLL_WRITE and !VM_WRITE situation.
> 
>>
>>>
>>> Hmm maybe I'll soften on this anon_vma idea then. Maybe it is a 'cheap fix' to
>>> rule out the _usual_ cases.
>>
>> Yeah, something to evaluate.
> 
> I'm thinking more and more we're probably actually safe with !vma->anon_vma ||
> !(vma->vm_flags & VM_MAYWRITE).

I think there are possible races, the question is how much you care 
about them.

In case of CoW-unsharing, you're not actually discarding data, because 
the page in the anon folio is to maintain a copy of the pagecache page 
(of course, they can go out of sync, but that's a different discussion).

-- 
Cheers,

David / dhildenb