Subject: Re: [RFC v3 07/22] landlock: Handle file comparisons

From: Mickaël Salaün <[email-masked]>
Date: Thu, 15 Sep 2016 01:02:22 +0200

On 14/09/2016 23:06, Alexei Starovoitov wrote:
> On Wed, Sep 14, 2016 at 09:24:00AM +0200, Mickaël Salaün wrote:
>> Add eBPF functions to compare file system access with a Landlock file
>> system handle:
>> * bpf_landlock_cmp_fs_prop_with_struct_file(prop, map, map_op, file)
>>   This function allows to compare the dentry, inode, device or mount
>>   point of the currently accessed file, with a reference handle.
>> * bpf_landlock_cmp_fs_beneath_with_struct_file(opt, map, map_op, file)
>>   This function allows an eBPF program to check if the current accessed
>>   file is the same or in the hierarchy of a reference handle.
>>
>> The goal of file system handle is to abstract kernel objects such as a
>> struct file or a struct inode. Userland can create this kind of handle
>> thanks to the BPF_MAP_UPDATE_ELEM command. The element is a struct
>> landlock_handle containing the handle type (e.g.
>> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) and a file descriptor. This could
>> also be any descriptions able to match a struct file or a struct inode
>> (e.g. path or glob string).
>>
>> Changes since v2:
>> * add MNT_INTERNAL check to only add file handle from user-visible FS
>>   (e.g. no anonymous inode)
>> * replace struct file* with struct path* in map_landlock_handle
>> * add BPF protos
>> * fix bpf_landlock_cmp_fs_prop_with_struct_file()
>>
>> Signed-off-by: Mickaël Salaün <[email-masked]>
>> Cc: Alexei Starovoitov <[email-masked]>
>> Cc: Andy Lutomirski <[email-masked]>
>> Cc: Daniel Borkmann <[email-masked]>
>> Cc: David S. Miller <[email-masked]>
>> Cc: James Morris <[email-masked]>
>> Cc: Kees Cook <[email-masked]>
>> Cc: Serge E. Hallyn <[email-masked]>
>> Link: https://lkml.kernel.org/r/[email-masked]
> 
> thanks for keeping the links to the previous discussion.
> Long term it should help, though I worry we already at the point
> where there are too many outstanding issues to resolve before we
> can proceed with reasonable code review.
> 
>> +/*
>> + * bpf_landlock_cmp_fs_prop_with_struct_file
>> + *
>> + * Cf. include/uapi/linux/bpf.h
>> + */
>> +static inline u64 bpf_landlock_cmp_fs_prop_with_struct_file(u64 r1_property,
>> +		u64 r2_map, u64 r3_map_op, u64 r4_file, u64 r5)
>> +{
>> +	u8 property = (u8) r1_property;
>> +	struct bpf_map *map = (struct bpf_map *) (unsigned long) r2_map;
>> +	enum bpf_map_array_op map_op = r3_map_op;
>> +	struct file *file = (struct file *) (unsigned long) r4_file;
> 
> please use just added BPF_CALL_ macros. They will help readability of the above.
> 
>> +	struct bpf_array *array = container_of(map, struct bpf_array, map);
>> +	struct path *p1, *p2;
>> +	struct map_landlock_handle *handle;
>> +	int i;
>> +
>> +	/* ARG_CONST_PTR_TO_LANDLOCK_HANDLE_FS is an arraymap */
>> +	if (unlikely(!map)) {
>> +		WARN_ON(1);
>> +		return -EFAULT;
>> +	}
>> +	if (unlikely(!file))
>> +		return -ENOENT;
>> +	if (unlikely((property | _LANDLOCK_FLAG_FS_MASK) != _LANDLOCK_FLAG_FS_MASK))
>> +		return -EINVAL;
>> +
>> +	/* for now, only handle OP_OR */
>> +	switch (map_op) {
>> +	case BPF_MAP_ARRAY_OP_OR:
>> +		break;
>> +	case BPF_MAP_ARRAY_OP_UNSPEC:
>> +	case BPF_MAP_ARRAY_OP_AND:
>> +	case BPF_MAP_ARRAY_OP_XOR:
>> +	default:
>> +		return -EINVAL;
>> +	}
>> +	p2 = &file->f_path;
>> +
>> +	synchronize_rcu();
> 
> that is completely broken.
> bpf programs are executing under rcu_lock.
> please enable CONFIG_PROVE_RCU and retest everything.

Thanks for the tip. I will fix this.

> 
> I would suggest for the next RFC to do minimal 7 patches up to this point
> with simple example that demonstrates the use case.
> I would avoid all unpriv stuff and all of seccomp for the next RFC as well,
> otherwise I don't think we can realistically make forward progress, since
> there are too many issues raised in the subsequent patches.

I hope we will find a common agreement about seccomp vs cgroup… I think
both approaches have their advantages, can be complementary and nicely
combined.

Unprivileged sandboxing is the main goal of Landlock. This should not be
a problem, even for privileged features, thanks to the new subtype/access.

> 
> The common part that is mergeable is prog's subtype extension to
> the verifier that can be used for better tracing and is the common
> piece of infra needed for both landlock and checmate LSMs
> (which must be one LSM anyway)

Agreed. With this RFC, the Checmate features (i.e. network helpers)
should be able to sit on top of Landlock.