Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, Feb 28, 2017 at 12:43 PM, Willem de Bruijn
[off-list ref] wrote:

On Tue, Feb 28, 2017 at 2:46 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Mon, Feb 27, 2017 at 10:57 AM, Michael Kerrisk
[off-list ref] wrote:

quoted

[CC += linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org]

Hi Willem

quoted

quoted

On a send call with MSG_ZEROCOPY, the kernel pins the user pages and
creates skbuff fragments directly from these pages. On tx completion,
it notifies the socket owner that it is safe to modify memory by
queuing a completion notification onto the socket error queue.

What happens if the user writes to the pages while it's not safe?

How about if you're writing to an interface or a route that has crypto
involved and a malicious user can make the data change in the middle
of a crypto operation, thus perhaps leaking the entire key?  (I
wouldn't be at all surprised if a lot of provably secure AEAD
constructions are entirely compromised if an attacker can get the
ciphertext and tag computed from a message that changed during the
computation.

Operations that read or write payload, such as this crypto example,
but also ebpf in tc or iptables, for instance, demand a deep copy using
skb_copy_ubufs before the operation.

This blacklist approach requires caution, but these paths should be
few and countable. It is not possible to predict at the socket layer
whether a packet will encounter any such operation, so white-listing
a subset of end-to-end paths is not practical.

How about hardware that malfunctions if the packet changes out from
under it?  A whitelist seems quite a bit safer.

--Andy