Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

David Hildenbrand
2023-04-28T15:14:14Z

[...]

>> This change has the potential to break existing setups. Simple example:
>> libvirt domains configured for file-backed VM memory that also has a vfio
>> device configured. It can easily be configured by users (evolving VM
>> configuration, copy-paste etc.). And it works from a VM perspective, because
>> the guest memory is essentially stale once the VM is shutdown and the pages
>> were unpinned. At least we're not concerned about stale data on disk.
>>
>> With your changes, such VMs would no longer start, breaking existing user
>> setups with a kernel update.
>
> Which vfio vm_ops are we talking about? vfio_pci_mmap_ops for example
> doesn't specify page_mkwrite or pfn_mkwrite. Unless you mean some arbitrary
> file system in the guest?

Sorry, you define a VM to have its memory backed by VM memory and, at
the same time, define a vfio-pci device for your VM, which will end up
long-term pinning the VM memory.

>
> I may well be missing context on this so forgive me if I'm being a little
> dumb here, but it'd be good to get a specific example.

I was giving to little details ;)

[...]

>>
>> I know, Jason und John will disagree, but I don't think we want to be very
>> careful with changing the default.
>>
>> Sure, we could warn, or convert individual users using a flag (io_uring).
>> But maybe we should invest more energy on a fix?
>
> This is proactively blocking a cleanup (eliminating vmas) that I believe
> will be useful in moving things forward. I am not against an opt-in option
> (I have been responding to community feedback in adapting my approach),
> which is the way I implemented it all the way back then :)

There are alternatives: just use a flag as Jason initially suggested and
use that in io_uring code. Then, you can also bail out on the GUP-fast
path as "cannot support it right now, never do GUP-fast".

IMHO, this patch is not a prereq.

>
> But given we know this is both entirely broken and a potential security
> issue, and FOLL_LONGTERM is about as egregious as you can get (user
> explicitly saying they'll hold write access indefinitely) I feel it is an
> important improvement and makes clear that this is not an acceptable usage.
>
> I see Jason has said more on this also :)
>
>>
>>
>>
>>
>>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>>> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
>>> ---
>>> include/linux/mm.h | 1 +
>>> mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>>> mm/mmap.c | 36 +++++++++++++++++++++++++++---------
>>> 3 files changed, 68 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 37554b08bb28..f7da02fc89c6 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -2433,6 +2433,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
>>> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
>>> MM_CP_UFFD_WP_RESOLVE)
>>>
>>> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
>>> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
>>> static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
>>> {
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 1f72a717232b..d36a5db9feb1 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -959,16 +959,51 @@ static int faultin_page(struct vm_area_struct *vma,
>>> return 0;
>>> }
>>>
>>> +/*
>>> + * Writing to file-backed mappings which require folio dirty tracking using GUP
>>> + * is a fundamentally broken operation, as kernel write access to GUP mappings
>>> + * do not adhere to the semantics expected by a file system.
>>> + *
>>> + * Consider the following scenario:-
>>> + *
>>> + * 1. A folio is written to via GUP which write-faults the memory, notifying
>>> + * the file system and dirtying the folio.
>>> + * 2. Later, writeback is triggered, resulting in the folio being cleaned and
>>> + * the PTE being marked read-only.
>>> + * 3. The GUP caller writes to the folio, as it is mapped read/write via the
>>> + * direct mapping.
>>> + * 4. The GUP caller, now done with the page, unpins it and sets it dirty
>>> + * (though it does not have to).
>>> + *
>>> + * This results in both data being written to a folio without writenotify, and
>>> + * the folio being dirtied unexpectedly (if the caller decides to do so).
>>> + */
>>> +static bool writeable_file_mapping_allowed(struct vm_area_struct *vma,
>>> + unsigned long gup_flags)
>>> +{
>>> + /* If we aren't pinning then no problematic write can occur. */
>>> + if (!(gup_flags & (FOLL_GET | FOLL_PIN)))
>>> + return true;
>>
>> FOLL_LONGTERM only applies to FOLL_PIN. This check can be dropped.
>
> I understand that of course (well maybe not of course, but I mean I do, I
> have oodles of diagrams referencing this int he book :) This is intended to
> document the fact that the check isn't relevant if we don't pin at all,
> e.g. reading this you see:-
>
> - (implicit) if not writing or anon we're good
> - if not pin we're good
> - ok we are only currently checking one especially egregious case
> - finally, perform the dirty tracking check.
>
> So this is intentional.
>
>>
>>> +
>>> + /* We limit this check to the most egregious case - a long term pin. */
>>> + if (!(gup_flags & FOLL_LONGTERM))
>>> + return true;
>>> +
>>> + /* If the VMA requires dirty tracking then GUP will be problematic. */
>>> + return vma_needs_dirty_tracking(vma);
>>> +}
>>> +
>>> static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>>> {
>>> vm_flags_t vm_flags = vma->vm_flags;
>>> int write = (gup_flags & FOLL_WRITE);
>>> int foreign = (gup_flags & FOLL_REMOTE);
>>> + bool vma_anon = vma_is_anonymous(vma);
>>>
>>> if (vm_flags & (VM_IO | VM_PFNMAP))
>>> return -EFAULT;
>>>
>>> - if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
>>> + if ((gup_flags & FOLL_ANON) && !vma_anon)
>>> return -EFAULT;
>>>
>>> if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
>>> @@ -978,6 +1013,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>>> return -EFAULT;
>>>
>>> if (write) {
>>> + if (!vma_anon &&
>>> + !writeable_file_mapping_allowed(vma, gup_flags))
>>> + return -EFAULT;
>>> +
>>> if (!(vm_flags & VM_WRITE)) {
>>> if (!(gup_flags & FOLL_FORCE))
>>> return -EFAULT;
>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>> index 536bbb8fa0ae..7b6344d1832a 100644
>>> --- a/mm/mmap.c
>>
>>
>> I'm probably missing something, why don't we have to handle GUP-fast (having
>> said that, it's hard to handle ;) )? The sequence you describe above should
>> apply to GUP-fast as well, no?
>>
>> 1) Pin writable mapped page using GUP-fast
>> 2) Trigger writeback
>> 3) Write to page via pin
>> 4) Unpin and set dirty
>
> You're right, and this is an excellent point. I worry about other GUP use
> cases too, but we're a bit out of luck there because we don't get to check
> the VMA _at all_ (which opens yet another Pandora's box about how safe it
> is to do unlocked pinning :)
>
> But again, this comes down to the fact we're trying to make things
> _incrementally__ better rather than throwing our hands up and saying one
> day my ship will come in...

That's not how security fixes are supposed to work IMHO, sorry.

--
Thanks,

David / dhildenb