[PATCH v2] mm/mremap: Fail map duplication attempts for private mappings

[Mike Kravetz]

mremap will create a 'duplicate' mapping if old_size == 0 is
specified.  Such duplicate mappings make no sense for private
mappings.  If duplication is attempted for a private mapping,
mremap creates a separate private mapping unrelated to the
original mapping and makes no modifications to the original.
This is contrary to the purpose of mremap which should return
a mapping which is in some way related to the original.

Therefore, return EINVAL in the case where if 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 | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..949f6a7 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -383,6 +383,15 @@ 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 a mapping is 'duplicated'.
+	 * Do not allow this for private mappings.
+	 */
+	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