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

[David Hildenbrand (Arm)]

From: Chris Mason <mason at kernel.org>

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 at vger.kernel.org
Signed-off-by: Chris Mason <mason at kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
[ david: simplify comment and description ]
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/memory.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 0f4db5080e371..ec7a1397ab833 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -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);

-- 
2.43.0