Re: [PATCH nf-next 0/3] netfilter: nf_tables: reject loads from uninitialized registers

[Phil Sutter <phil@nwl.cc>]

On Fri, May 05, 2023 at 03:46:55PM +0200, Florian Westphal wrote:
> Phil Sutter <phil@nwl.cc> wrote:
> > On Fri, May 05, 2023 at 01:16:53PM +0200, Florian Westphal wrote:
> > > Keep a per-rule bitmask that tracks registers that have seen a store,
> > > then reject loads when the accessed registers haven't been flagged.
> > > 
> > > This changes uabi contract, because we previously allowed this.
> > > Neither nftables nor iptables-nft create such rules.
> > 
> > Did you consider keeping this bitmask on a per base-chain level? One had
> > to perform this for each base chain of a table upon each rule change and
> > traverse the tree of chains jumped to from there. I guess the huge
> > overhead disqualifies this, though.
> 
> Yes, but its very hard task, because in that case we also need to prove
> that a write *WILL* happen, rather than *might happen*.
> 
> Consider:
> 
> rule1:
> ip protocol tcp iifname "eth0" ...
> reg1 := ip protocol
> cmp reg1
> reg2 := meta iifname
> 
> rule2:
> iifname "eth1" ...
> cmp reg2 "eth0"
> 
> rule 2 has to be rejected because reg2 might be unitialized for != tcp.
> 
> Even if we can handle this some way, we now also need to revalidate the
> ruleset on deletes, because we'd have to detect when a register write
> we depend on goes away.

Ah, right. I forgot about "partial" rule execution again. Same thing
which broke expression reduction for us.

Maybe one could introduce a "chain optimizer" creating an initial
meta-rule which just populates registers with packet/meta data rules
may need. Not something I would want to rely upon regarding kernel info
leaks, though.

Cheers, Phil