Re: [PATCH 34/46] mm/memory: reject remote COW faults on PFN mappings

[Pedro Falcato]

On Tue, Oct 27, 2026 at 12:17:54AM +0100, David Hildenbrand (Arm) wrote:
> From: Chris Mason <mason@xxxxxxxxxx>
> 
> When handling a COW fault without a normal source page (PFN mappings),
> __wp_page_copy_user() copies from the faulting user address. For remote
> faults, this accesses the wrong address space, resulting in wrong data
> getting copied.
> 
> For example, user_events updates an enable bit in the registering
> process's memory. If that memory needs to be faulted in,
> user_event_enabler_fault_fixup() calls fixup_user_fault() from a work
> item. Placing the enable bit in a private PFN mapping can trigger this
> remote COW path.
> 
> Reject these faults with VM_FAULT_SIGBUS when the target mm differs from
> current->mm: remote faults. Ordinary (non-PFN) mappings are not affected.
> 
> The user-address copy dates back to commit 6aab341e0a28 ("mm: re-architect
> the VM_UNPAGED logic"); likely, actual ways to get remote faults in
> there on COW PFN mappings appeared afterwards. But the fix is simple and
> straight forward.
> 
> Fix generated by an LLM and sent by Chris Mason in private.
> 
> Fixes: 6aab341e0a28 ("mm: re-architect the VM_UNPAGED logic")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> [ david: simplify comment and description ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx> 

-- 
Pedro