FAILED: patch "[PATCH] mm/memory: reject remote COW faults on PFN mappings" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 8db6921cc07b74197a3866d62ed262b68aeadcf3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 8db6921cc07b74197a3866d62ed262b68aeadcf3 Mon Sep 17 00:00:00 2001
From: Chris Mason <[email protected]>
Date: Sat, 31 Oct 2026 10:40:04 +0100
Subject: [PATCH] mm/memory: reject remote COW faults on PFN mappings

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.

---
 mm/memory.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 5392bcb1d24d6..ae8d567bc6ba3 100644
@@ -3761,6 +3761,13 @@ static inline int __wp_page_copy_user(struct page *dst, struct page *src,
 		return 0;
 	}
 
+	/*
+	 * __copy_from_user_inatomic() only works as expected for local faults.
+	 * Reject remote faults.
+	 */
+	if (unlikely(mm != current->mm))
+		return -EFAULT;
+
 	/*
 	 * If the source page was a PFN mapping, we don't have
 	 * a "struct page" for it. We do a best-effort copy by

@@ -4086,6 +4093,8 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 				folio_put(old_folio);
 
 			delayacct_wpcopy_end();
+			if (err == -EFAULT)
+				return VM_FAULT_SIGBUS;
 			return err == -EHWPOISON ? VM_FAULT_HWPOISON : 0;
 		}
 		kmsan_copy_page_meta(&new_folio->page, vmf->page);

2.53.0