netdev - Re: [PATCH RFC bpf-next 0/4] bpf: Add support for XDP programs in DEVMAPs

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Date:   Mon, 25 May 2020 14:56:26 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        brouer@...hat.com
Subject: Re: [PATCH RFC bpf-next 0/4] bpf: Add support for XDP programs in DEVMAPs

Jesper Dangaard Brouer <brouer@...hat.com> writes:

> On Mon, 25 May 2020 14:15:32 +0200
> Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
>> David Ahern <dsahern@...il.com> writes:
>> 
>> > On 5/22/20 9:59 AM, Toke Høiland-Jørgensen wrote:  
>> >> David Ahern <dsahern@...nel.org> writes:
>> >>   
>> >>> Implementation of Daniel's proposal for allowing DEVMAP entries to be
>> >>> a device index, program id pair. Daniel suggested an fd to specify the
>> >>> program, but that seems odd to me that you insert the value as an fd, but
>> >>> read it back as an id since the fd can be closed.  
>> >> 
>> >> While I can be sympathetic to the argument that it seems odd, every
>> >> other API uses FD for insert and returns ID, so why make it different
>> >> here? Also, the choice has privilege implications, since the CAP_BPF
>> >> series explicitly makes going from ID->FD a more privileged operation
>> >> than just querying the ID.
>
> Sorry, I don't follow.
> Can someone explain why is inserting an ID is a privilege problem?

See description here:
https://lore.kernel.org/bpf/20200513230355.7858-1-alexei.starovoitov@gmail.com/

Specifically, this part:

> Consolidating all CAP checks at load time makes security model similar to
> open() syscall. Once the user got an FD it can do everything with it.
> read/write/poll don't check permissions. The same way when bpf_prog_load
> command returns an FD the user can do everything (including attaching,
> detaching, and bpf_test_run).
> 
> The important design decision is to allow ID->FD transition for
> CAP_SYS_ADMIN only. What it means that user processes can run
> with CAP_BPF and CAP_NET_ADMIN and they will not be able to affect each
> other unless they pass FDs via scm_rights or via pinning in bpffs.


>> > I do not like the model where the kernel changes the value the user
>> > pushed down.  
>> 
>> Yet it's what we do in every other interface where a user needs to
>> supply a program, including in prog array maps. So let's not create a
>> new inconsistent interface here...
>
> I sympathize with Ahern on this.  It seems very weird to insert/write
> one value-type, but read another value-type.

Yeah, I do kinda agree that it's a bit weird. But it's what we do
everywhere else, so I think consistency wins out here. There might be an
argument that maps should be different (because they're conceptually a
read/write data structure not a syscall return value). But again, we
already have a map type that takes prog IDs, and that already does the
rewriting, so doing it different here would be even weirder...

-Toke