Re: [RFC PATCH v2 0/3] eliminate mmap() retry merge, add .mmap_prepare hook

[Christian Brauner <brauner@kernel.org>]

On Fri, May 02, 2025 at 01:59:49PM +0100, Lorenzo Stoakes wrote:
> On Fri, May 02, 2025 at 02:20:38PM +0200, Jan Kara wrote:
> > On Thu 01-05-25 18:25:26, Lorenzo Stoakes wrote:
> > > During the mmap() of a file-backed mapping, we invoke the underlying driver
> > > file's mmap() callback in order to perform driver/file system
> > > initialisation of the underlying VMA.
> > >
> > > This has been a source of issues in the past, including a significant
> > > security concern relating to unwinding of error state discovered by Jann
> > > Horn, as fixed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> > > error path behaviour") which performed the recent, significant, rework of
> > > mmap() as a whole.
> > >
> > > However, we have had a fly in the ointment remain - drivers have a great
> > > deal of freedom in the .mmap() hook to manipulate VMA state (as well as
> > > page table state).
> > >
> > > This can be problematic, as we can no longer reason sensibly about VMA
> > > state once the call is complete (the ability to do - anything - here does
> > > rather interfere with that).
> > >
> > > In addition, callers may choose to do odd or unusual things which might
> > > interfere with subsequent steps in the mmap() process, and it may do so and
> > > then raise an error, requiring very careful unwinding of state about which
> > > we can make no assumptions.
> > >
> > > Rather than providing such an open-ended interface, this series provides an
> > > alternative, far more restrictive one - we expose a whitelist of fields
> > > which can be adjusted by the driver, along with immutable state upon which
> > > the driver can make such decisions:
> > >
> > > struct vm_area_desc {
> > > 	/* Immutable state. */
> > > 	struct mm_struct *mm;
> > > 	unsigned long start;
> > > 	unsigned long end;
> > >
> > > 	/* Mutable fields. Populated with initial state. */
> > > 	pgoff_t pgoff;
> > > 	struct file *file;
> > > 	vm_flags_t vm_flags;
> > > 	pgprot_t page_prot;
> > >
> > > 	/* Write-only fields. */
> > > 	const struct vm_operations_struct *vm_ops;
> > > 	void *private_data;
> > > };
> > >
> > > The mmap logic then updates the state used to either merge with a VMA or
> > > establish a new VMA based upon this logic.
> > >
> > > This is achieved via new file hook .mmap_prepare(), which is, importantly,
> > > invoked very early on in the mmap() process.
> > >
> > > If an error arises, we can very simply abort the operation with very little
> > > unwinding of state required.
> >
> > Looks sensible. So is there a plan to transform existing .mmap hooks to
> > .mmap_prepare hooks? I agree that for most filesystems this should be just
> > easy 1:1 replacement and AFAIU this would be prefered?
> 
> Thanks!
> 
> Yeah the intent is to convert _all_ callers away from .mmap() so we can
> lock down what drivers are doing and be able to (relatively) safely make
> assumptions about what's going on in mmap logic.
> 
> As David points out, we may need to add new callbacks to account for other

The plural is a little worrying, let's please aim minimize the number of
new methods we need for this.