linux-kernel - Re: [RFC v3 07/22] landlock: Handle file comparisons

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Message-ID: <20160914190723.GB5617@pc.thejh.net>
Date:   Wed, 14 Sep 2016 21:07:23 +0200
From:   Jann Horn <jann@...jh.net>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Mack <daniel@...que.org>,
        David Drysdale <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Paul Moore <pmoore@...hat.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>, Tejun Heo <tj@...nel.org>,
        Will Drewry <wad@...omium.org>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org, netdev@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [RFC v3 07/22] landlock: Handle file comparisons

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.
[...]
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 94256597eacd..edaab4c87292 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -603,6 +605,9 @@ static void landlock_put_handle(struct map_landlock_handle *handle)
>  	enum bpf_map_handle_type handle_type = handle->type;
>  
>  	switch (handle_type) {
> +	case BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD:
> +		path_put(&handle->path);
> +		break;
>  	case BPF_MAP_HANDLE_TYPE_UNSPEC:
>  	default:
>  		WARN_ON(1);
[...]
> diff --git a/security/landlock/checker_fs.c b/security/landlock/checker_fs.c
> new file mode 100644
> index 000000000000..39eb85dc7d18
> --- /dev/null
> +++ b/security/landlock/checker_fs.c
[...]
> +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;
> +	struct bpf_array *array = container_of(map, struct bpf_array, map);
> +	struct path *p1, *p2;
> +	struct map_landlock_handle *handle;
> +	int i;

Please don't use int when iterating over an array, use size_t.


> +	/* for now, only handle OP_OR */

Is "OP_OR" an appropriate name for something that ANDs the success of
checks?


[...]
> +	synchronize_rcu();

Can you put a comment here that explains what's going on?


> +	for (i = 0; i < array->n_entries; i++) {
> +		bool result_dentry = !(property & LANDLOCK_FLAG_FS_DENTRY);
> +		bool result_inode = !(property & LANDLOCK_FLAG_FS_INODE);
> +		bool result_device = !(property & LANDLOCK_FLAG_FS_DEVICE);
> +		bool result_mount = !(property & LANDLOCK_FLAG_FS_MOUNT);
> +
> +		handle = (struct map_landlock_handle *)
> +				(array->value + array->elem_size * i);
> +
> +		if (handle->type != BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) {
> +			WARN_ON(1);
> +			return -EFAULT;
> +		}
> +		p1 = &handle->path;
> +
> +		if (!result_dentry && p1->dentry == p2->dentry)
> +			result_dentry = true;

Why is this safe? As far as I can tell, this is not in an RCU read-side
critical section (synchronize_rcu() was just called), and no lock has been
taken. What prevents someone from removing the arraymap entry while we're
looking at it? Am I missing something?


[...]
> +static inline u64 bpf_landlock_cmp_fs_beneath_with_struct_file(u64 r1_option,
> +		u64 r2_map, u64 r3_map_op, u64 r4_file, u64 r5)
> +{
> +	u8 option = (u8) r1_option;
> +	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;
> +	struct bpf_array *array = container_of(map, struct bpf_array, map);
> +	struct path *p1, *p2;
> +	struct map_landlock_handle *handle;
> +	int i;

As above, please use size_t.

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