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/cb9d9f6a-7095-582f-15a5-62643d65c736@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index 3f23715d3c69..7395564daa6c 100644
--- 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);