[RFC PATCH v2 00/12] Socket type control for Landlock

[Mikhail Ivanov]

6/4/2024 11:22 PM, Günther Noack wrote:
> On Fri, May 24, 2024 at 05:30:03PM +0800, Mikhail Ivanov wrote:
>> Hello! This is v2 RFC patch dedicated to socket protocols restriction.
>>
>> It is based on the landlock's mic-next branch on top of v6.9 kernel
>> version.
> 
> Hello Mikhail!
> 
> I patched in your patchset and tried to use the feature with a small
> demo tool, but I ran into what I think is a bug -- do you happen to
> know what this might be?
> 
> I used 6.10-rc1 as a base and patched your patches on top.
> 
> The code is a small tool called "nonet", which does the following:
> 
>    - Disable socket creation with a Landlock ruleset with the following
>      attributes:
>    
>      struct landlock_ruleset_attr attr = {
>        .handled_access_socket = LANDLOCK_ACCESS_SOCKET_CREATE,
>      };
> 
>    - open("/dev/null", O_WRONLY)
> 
> Expected result:
> 
>    - open() should work
> 
> Observed result:
> 
>    - open() fails with EACCES.
> 
> I traced this with perf, and found that the open() gets rejected from
> Landlock's hook_file_open, whereas hook_socket_create does not get
> invoked.  This is surprising to me -- Enabling a policy for socket
> creation should not influence the outcome of opening files!
> 
> Tracing commands:
> 
>    sudo perf probe hook_socket_create '$params'
>    sudo perf probe 'hook_file_open%return $retval'
>    sudo perf record -e 'probe:*' -g -- ./nonet
>    sudo perf report
>   
> You can find the tool in my landlock-examples repo in the nonet_bug branch:
> https://github.com/gnoack/landlock-examples/blob/nonet_bug/nonet.c
> 
> Landlock is enabled like this:
> https://github.com/gnoack/landlock-examples/blob/nonet_bug/sandbox_socket.c
> 
> Do you have a hunch what might be going on?

Hello Günther!
Big thanks for this research!

I figured out that I define LANDLOCK_SHIFT_ACCESS_SOCKET macro in
really strange way (see landlock/limits.h):

   #define LANDLOCK_SHIFT_ACCESS_SOCKET	LANDLOCK_NUM_ACCESS_SOCKET

With this definition, socket access mask overlaps the fs access
mask in ruleset->access_masks[layer_level]. That's why
landlock_get_fs_access_mask() returns non-zero mask in hook_file_open().

So, the macro must be defined in this way:

   #define LANDLOCK_SHIFT_ACCESS_SOCKET	(LANDLOCK_NUM_ACCESS_NET +
                                          LANDLOCK_NUM_ACCESS_FS)

With this fix, open() doesn't fail in your example.

I'm really sorry that I somehow made such a stupid typo. I will try my
best to make sure this doesn't happen again.

> 
> Thanks,
> –Günther
>