[PATCH 04/39] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

When a user requests an mmap_action be performed in mmap_prepare, this
involves populating the VMA range with data.

However, if the VMA is mergeable, it might then mistakenly be merged with
another VMA without having populated the range.

Every mmap action currently available sets VMA flags such that the VMA
cannot be merged.

However, to ensure that no future mmap action falls foul of this, assert
that this is the case upon mmap_prepare validation.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/vma.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/vma.c b/mm/vma.c
index a24f04428580..9e45fc83f74c 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2794,6 +2794,15 @@ static int mmap_validate(unsigned long prev_start,
 int mmap_prepare_validate(const struct vm_area_desc *prev_desc,
 			  const struct vm_area_desc *desc)
 {
+	/*
+	 * It is not valid to execute mmap actions for VMAs which can be merged,
+	 * as any such merge would leave portions of the mapping incorrectly
+	 * unmapped.
+	 */
+	if (vma_flags_can_merge(&desc->vma_flags) &&
+	    WARN_ON_ONCE(desc->action.type != MMAP_NOTHING))
+		return -EINVAL;
+
 	return mmap_validate(prev_desc->start, desc->start,
 			     &prev_desc->vma_flags, &desc->vma_flags);
 }

-- 
2.55.0