It makes no sense for a mapping whose contents the kernel does not own to
specify that the range is MMIO.

Prior to this patch, all in-tree drivers which did so have been updated
such that they are marked as kernel-owned. The check WARNs and fails the
mmap for any out-of-tree driver that still sets VMA_IO_BIT without a kernel
mapping.

No functional change intended for in-tree code.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/vma.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/vma.c b/mm/vma.c
index cb0c4c625756..5996757d5aaf 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2772,6 +2772,12 @@ static int mmap_validate_vma_flags(const vma_flags_t *flags)
 		return -EINVAL;
 #endif
 
+	if (!vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT)) {
+		/* Only kernel-owned mappings may set VMA_IO_BIT. */
+		if (WARN_ON_ONCE(vma_flags_test(flags, VMA_IO_BIT)))
+			return -EINVAL;
+	}
+
 	return 0;
 }
 

-- 
2.55.0