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

[Mike Kravetz]

On 07/13/2017 12:11 PM, Vlastimil Babka wrote:
> [+CC linux-api]
> 
> On 07/13/2017 05:58 PM, Mike Kravetz wrote:
>> 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.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 

In another e-mail thread, Andrea makes the case that mremap(old_size == 0)
of private file backed mappings could possibly be used for something useful.
For example to create a private COW mapping.  Of course, a better way to do
this would be simply using the fd to create a private mapping.

If returning EINVAL for all private mappings is too general, the following
patch adds a check to only return EINVAL for private anon mappings.

mm/mremap: Fail map duplication attempts for private anon mappings

mremap will create a 'duplicate' mapping if old_size == 0 is
specified.  Such duplicate mappings make no sense for private
anonymous mappings.  If duplication is attempted for a private
anon 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 an attempt is made to
duplicate a private anon mapping.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/mremap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..586ea3d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -383,6 +383,14 @@ 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 anon mappings.
+	 */
+	if (!old_len && vma_is_anonymous(vma) &&
+	    !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
+		return ERR_PTR(-EINVAL);
+
 	if (is_vm_hugetlb_page(vma))
 		return ERR_PTR(-EINVAL);
 
-- 
2.7.5