Re: [PATCH] mm/gup: fix NULL pointer dereference in fixup_user_fault()

[Lance Yang <lance.yang@linux.dev>]

On Sun, Oct 04, 2026 at 12:56:01PM -0700, Andrew Morton wrote:
>On Sun,  4 Oct 2026 02:48:46 +0700 Nguyen Duy Nhat Anh <neganhat@gmail.com> wrote:
>
>> In fixup_user_fault(), the 'unlocked' parameter is checked for NULL
>> early on, allowing callers to pass NULL if they do not track whether the
>> mmap lock was dropped.
>> 
>> However, if handle_mm_fault() returns VM_FAULT_COMPLETED, line 1597
>> dereferences 'unlocked' directly (*unlocked = true) without checking
>> if it is NULL. Callers like s390's pci_mmio.c pass NULL for 'unlocked',
>> leading to a kernel NULL pointer dereference when VM_FAULT_COMPLETED
>> occurs.
>> 
>> Fix this by checking if 'unlocked' is non-NULL before assigning to it.
>
>This code is too subtle so you aren't the first to attempt to "fix" it.
>
>The key hint is in the kerneldoc:
>
> * @unlocked:	did we unlock the mmap_lock while retrying, maybe NULL if caller
> *		does not allow retry. If NULL, the caller must guarantee
> *		that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY.
>
>Trace through the
>FAULT_FLAG_ALLOW_RETRY/VM_FAULT_COMPLETED/VM_FAULT_RETRY logic
>to confirm that the null deref is a cant-happen.

IIUC, that's indeed a can't-happen :)