[PATCH] mm/mremap: Fail map duplication attempts for private mappings

[Mike Kravetz]

mremap will attempt to create a 'duplicate' mapping if old_size
== 0 is specified.  In the case of private mappings, mremap
will actually create a fresh separate private mapping unrelated
to the original.  This does not fit with the design semantics of
mremap as the intention is to create a new mapping based on the
original.

Therefore, return EINVAL in the case where an attempt is made
to duplicate a private mapping.  Also, print a warning message
(once) if such an attempt is made.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/mremap.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..75b167d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -383,6 +383,19 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (!vma || vma->vm_start > addr)
 		return ERR_PTR(-EFAULT);
 
+	/*
+	 * !old_len is a special case where an attempt is made to 'duplicate'
+	 * a mapping.  This makes no sense for private mappings as it will
+	 * instead create a fresh/new mapping unrelated to the original.  This
+	 * is contrary to the basic idea of mremap which creates new mappings
+	 * based on the original.  There are no known use cases for this
+	 * behavior.  As a result, fail such attempts.
+	 */
+	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
+		pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
+		return ERR_PTR(-EINVAL);
+	}
+
 	if (is_vm_hugetlb_page(vma))
 		return ERR_PTR(-EINVAL);
 
-- 
2.7.5