netdev - Re: [PATCH 2/3] security: bpf: Add eBPF LSM hooks and security field to eBPF map

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Date:   Thu, 31 Aug 2017 19:05:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Chenbo Feng <chenbofeng.kernel@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        linux-security-module@...r.kernel.org,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        netdev@...r.kernel.org, SELinux <Selinux@...ho.nsa.gov>,
        lorenzo@...gle.com, Chenbo Feng <fengc@...gle.com>
Subject: Re: [PATCH 2/3] security: bpf: Add eBPF LSM hooks and security field
 to eBPF map

On Thu, Aug 31, 2017 at 01:56:34PM -0700, Chenbo Feng wrote:
> From: Chenbo Feng <fengc@...gle.com>
> 
> Introduce a pointer into struct bpf_map to hold the security information
> about the map. The actual security struct varies based on the security
> models implemented. Place the LSM hooks before each of the unrestricted
> eBPF operations, the map_update_elem and map_delete_elem operations are
> checked by security_map_modify. The map_lookup_elem and map_get_next_key
> operations are checked by securtiy_map_read.
> 
> Signed-off-by: Chenbo Feng <fengc@...gle.com>

...

> @@ -410,6 +418,10 @@ static int map_lookup_elem(union bpf_attr *attr)
>  	if (IS_ERR(map))
>  		return PTR_ERR(map);
>  
> +	err = security_map_read(map);
> +	if (err)
> +		return -EACCES;
> +
>  	key = memdup_user(ukey, map->key_size);
>  	if (IS_ERR(key)) {
>  		err = PTR_ERR(key);
> @@ -490,6 +502,10 @@ static int map_update_elem(union bpf_attr *attr)
>  	if (IS_ERR(map))
>  		return PTR_ERR(map);
>  
> +	err = security_map_modify(map);

I don't feel these extra hooks are really thought through.
With such hook you'll disallow map_update for given map. That's it.
The key/values etc won't be used in such security decision.
In such case you don't need such hooks in update/lookup at all.
Only in map_creation and object_get calls where FD can be received.
In other words I suggest to follow standard unix practices:
Do permissions checks in open() and allow read/write() if FD is valid.
Same here. Do permission checks in prog_load/map_create/obj_pin/get
and that will be enough to jail bpf subsystem.
bpf cmds that need to be fast (like lookup and update) should not
have security hooks.