linux-kernel - Re: [RFC PATCH v2 0/3] eliminate mmap() retry merge, add .mmap

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Message-ID: <uevybgodhkny6dihezto4gkfup6n7znaei6q4ehlkksptlptwr@vgm2tzhpidli>
Date: Fri, 2 May 2025 14:20:38 +0200
From: Jan Kara <jack@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>, 
	Pedro Falcato <pfalcato@...e.de>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH v2 0/3] eliminate mmap() retry merge, add
 .mmap_prepare hook

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?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR