mm/mremap: fail map duplication attempts for private mappings

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.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Kravetz <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Aaron Lu <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/mremap.c b/mm/mremap.c
index 3f23715d3c692770193ff99e707131d95369065c..7395564daa6c228ab704be92b718112dbb2fb6aa 100644 (file)
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -384,6 +384,19 @@ 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);