On Jan 8, 2021, at 3:34 PM, Andrea Arcangeli [off-list ref] wrote:

On Fri, Jan 08, 2021 at 10:31:24AM -0800, Andy Lutomirski wrote:

quoted

Can we just remove vmsplice() support?  We could make it do a normal

quoted

copy, thereby getting rid of a fair amount of nastiness and potential
attacks.  Even ignoring issues relating to the length of time that the
vmsplice reference is alive, we also have whatever problems could be
caused by a malicious or misguided user vmsplice()ing some memory and
then modifying it.

Sorry to ask but I'm curious, what also goes wrong if the user
modifies memory under GUP pin from vmsplice? That's not obvious to
see.

It breaks the otherwise true rule that the data in pipe buffers is
immutable.  Even just quoting the manpage:

       SPLICE_F_GIFT
              The user pages are a gift to the kernel.   The  application  may
              not  modify  this  memory ever, otherwise the page cache and on-
              disk data may differ.

That's no good.

I can also imagine use cases in which modified vmsplice() pages that
end up in various parts of the network stack could be problematic.
For example, if you can arrange for TCP or, worse, TLS to transmit
data and then retransmit modified data, you might get odd results.  In
the latter case, some security properties of TLS might be broken.

--Andy