Re: [PATCH] device-dax: fail all private mapping attempts

[Dan Williams]

On Mon, Dec 5, 2016 at 5:01 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> On Wed, 16 Nov 2016, Dan Williams wrote:
>
>> The device-dax implementation originally tried to be tricky and allow
>> private read-only mappings, but in the process allowed writable
>> MAP_PRIVATE + MAP_NORESERVE mappings.  For simplicity and predictability
>> just fail all private mapping attempts since device-dax memory is
>> statically allocated and will never support overcommit.
>>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
>> Reported-by: Pawel Lebioda <pawel.lebioda@xxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  drivers/dax/dax.c |    4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
>> index 0e499bfca41c..3d94ff20fdca 100644
>> --- a/drivers/dax/dax.c
>> +++ b/drivers/dax/dax.c
>> @@ -270,8 +270,8 @@ static int check_vma(struct dax_dev *dax_dev, struct vm_area_struct *vma,
>>       if (!dax_dev->alive)
>>               return -ENXIO;
>>
>> -     /* prevent private / writable mappings from being established */
>> -     if ((vma->vm_flags & (VM_NORESERVE|VM_SHARED|VM_WRITE)) == VM_WRITE) {
>> +     /* prevent private mappings from being established */
>> +     if ((vma->vm_flags & VM_SHARED) != VM_SHARED) {
>
> I think that is more restrictive than you intended: haven't tried,
> but I believe it rejects a PROT_READ, MAP_SHARED, O_RDONLY fd mmap,
> leaving no way to mmap /dev/dax without write permission to it.
>
> See line 1393 of mm/mmap.c: the test you want is probably
>         if (!(vma->vm_flags & VM_MAYSHARE))
>

Yes, it is. Thank you!

Fix for the fix on the way...