linux-kernel - Re: [PATCH 1/2] mm/memory: ensure fork child sees coherent memory snapshot

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Message-ID: <aD8--plab38qiQF8@casper.infradead.org>
Date: Tue, 3 Jun 2025 19:29:14 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, linux-mm@...ck.org,
	Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/memory: ensure fork child sees coherent memory
 snapshot

On Tue, Jun 03, 2025 at 08:21:02PM +0200, Jann Horn wrote:
> When fork() encounters possibly-pinned pages, those pages are immediately
> copied instead of just marking PTEs to make CoW happen later. If the parent
> is multithreaded, this can cause the child to see memory contents that are
> inconsistent in multiple ways:
> 
> 1. We are copying the contents of a page with a memcpy() while userspace
>    may be writing to it. This can cause the resulting data in the child to
>    be inconsistent.
> 2. After we've copied this page, future writes to other pages may
>    continue to be visible to the child while future writes to this page are
>    no longer visible to the child.
> 
> This means the child could theoretically see incoherent states where
> allocator freelists point to objects that are actually in use or stuff like
> that. A mitigating factor is that, unless userspace already has a deadlock
> bug, userspace can pretty much only observe such issues when fancy lockless
> data structures are used (because if another thread was in the middle of
> mutating data during fork() and the post-fork child tried to take the mutex
> protecting that data, it might wait forever).

Um, OK, but isn't that expected behaviour?  POSIX says:

: A process shall be created with a single thread. If a multi-threaded
: process calls fork(), the new process shall contain a replica of the
: calling thread and its entire address space, possibly including the
: states of mutexes and other resources. Consequently, the application
: shall ensure that the child process only executes async-signal-safe
: operations until such time as one of the exec functions is successful.

It's always been my understanding that you really, really shouldn't call
fork() from a multithreaded process.