Re: [PATCH 09/19] mm: New follow_pfnmap API

Jason Gunthorpe
2024-08-14T22:14:54Z

On Wed, Aug 14, 2024 at 02:24:47PM -0400, Peter Xu wrote:
> On Wed, Aug 14, 2024 at 10:19:54AM -0300, Jason Gunthorpe wrote:
> > On Fri, Aug 09, 2024 at 12:08:59PM -0400, Peter Xu wrote:
> >
> > > +/**
> > > + * follow_pfnmap_start() - Look up a pfn mapping at a user virtual address
> > > + * @args: Pointer to struct @follow_pfnmap_args
> > > + *
> > > + * The caller needs to setup args->vma and args->address to point to the
> > > + * virtual address as the target of such lookup. On a successful return,
> > > + * the results will be put into other output fields.
> > > + *
> > > + * After the caller finished using the fields, the caller must invoke
> > > + * another follow_pfnmap_end() to proper releases the locks and resources
> > > + * of such look up request.
> > > + *
> > > + * During the start() and end() calls, the results in @args will be valid
> > > + * as proper locks will be held. After the end() is called, all the fields
> > > + * in @follow_pfnmap_args will be invalid to be further accessed.
> > > + *
> > > + * If the PTE maps a refcounted page, callers are responsible to protect
> > > + * against invalidation with MMU notifiers; otherwise access to the PFN at
> > > + * a later point in time can trigger use-after-free.
> > > + *
> > > + * Only IO mappings and raw PFN mappings are allowed.
> >
> > What does this mean? The paragraph before said this can return a
> > refcounted page?
>
> This came from the old follow_pte(), I kept that as I suppose we should
> allow VM_IO | VM_PFNMAP just like before, even if in this case I suppose
> only the pfnmap matters where huge mappings can start to appear.

If that is the intention it should actively block returning anything
that is vm_normal_page() not check the VM flags, see the other
discussion..

It makes sense as a restriction if you call the API follow pfnmap.

> > > + * The mmap semaphore
> > > + * should be taken for read, and the mmap semaphore cannot be released
> > > + * before the end() is invoked.
> >
> > This function is not safe for IO mappings and PFNs either, VFIO has a
> > known security issue to call it. That should be emphasised in the
> > comment.
>
> Any elaboration on this? I could have missed that..

Just because the memory is a PFN or IO doesn't mean it is safe to
access it without a refcount. There are many driver scenarios where
revoking a PFN from mmap needs to be a hard fence that nothing else
has access to that PFN. Otherwise it is a security problem for that
driver.

> I suppose so? As the pgtable is stable, I thought it means it's safe, but
> I'm not sure now when you mentioned there's a VFIO known issue, so I could
> have overlooked something. There's no address returned, but pfn, pgprot,
> write, etc.

zap/etc will wait on the PTL, I think, so it should be safe for at
least the issues I am thinking of.

> The user needs to do proper mapping if they need an usable address,
> e.g. generic_access_phys() does ioremap_prot() and recheck the pfn didn't
> change.

No, you can't take the phys_addr_t outside the start/end region that
explicitly holds the lock protecting it. This is what the comment must
warn against doing.

Jason