Re: [PATCH net-next v7 08/10] bpf: Add a Landlock sandbox example

["Mickaël Salaün" <mic@digikod.net>]

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On 24/08/2017 04:59, Alexei Starovoitov wrote:
> On Mon, Aug 21, 2017 at 02:09:31AM +0200, Mickaël Salaün wrote:
>> Add a basic sandbox tool to create a process isolated from some part of
>> the system. This sandbox create a read-only environment. It is only
>> allowed to write to a character device such as a TTY:
>>
>>   # :> X
>>   # echo $?
>>   0
>>   # ./samples/bpf/landlock1 /bin/sh -i
>>   Launching a new sandboxed process.
>>   # :> Y
>>   cannot create Y: Operation not permitted
>>
>> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> 
> ...
> 
>> +SEC("landlock1")
>> +static int landlock_fs_prog1(struct landlock_context *ctx)
>> +{
>> +	char fmt_error_mode[] = "landlock1: error: get_mode:%lld\n";
>> +	char fmt_error_access[] = "landlock1: error: access denied\n";
>> +	long long ret;
>> +
>> +	/*
>> +	 * The argument ctx->arg2 contains bitflags of actions for which the
>> +	 * rule is run.  The flag LANDLOCK_ACTION_FS_WRITE means that a write
>> +	 * is requested by one of the userspace processes restricted by this
>> +	 * rule. The following test allows any actions which does not include a
>> +	 * write.
>> +	 */
>> +	if (!(ctx->arg2 & LANDLOCK_ACTION_FS_WRITE))
>> +		return 0;
>> +
>> +	/*
>> +	 * The argument ctx->arg1 is a file handle for which the process want
>> +	 * to access. The function bpf_handle_fs_get_mode() return the mode of
>> +	 * a file (e.g. S_IFBLK, S_IFDIR, S_IFREG...). If there is an error,
>> +	 * for example if the argument is not a file handle, then an
>> +	 * -errno value is returned. Otherwise the caller get the file mode as
>> +	 *  with stat(2).
>> +	 */
>> +	ret = bpf_handle_fs_get_mode((void *)ctx->arg1);
>> +	if (ret < 0) {
>> +
>> +		/*
>> +		 * The bpf_trace_printk() function enable to write in the
>> +		 * kernel eBPF debug log, accessible through
>> +		 * /sys/kernel/debug/tracing/trace_pipe . To be allowed to call
>> +		 * this function, a Landlock rule must have the
>> +		 * LANDLOCK_SUBTYPE_ABILITY_DEBUG ability, which is only
>> +		 * allowed for CAP_SYS_ADMIN.
>> +		 */
>> +		bpf_trace_printk(fmt_error_mode, sizeof(fmt_error_mode), ret);
>> +		return 1;
>> +	}
>> +
>> +	/*
>> +	 * This check allows the action on the file if it is a directory or a
>> +	 * pipe. Otherwise, a message is printed to the eBPF log.
>> +	 */
>> +	if (S_ISCHR(ret) || S_ISFIFO(ret))
>> +		return 0;
>> +	bpf_trace_printk(fmt_error_access, sizeof(fmt_error_access));
>> +	return 1;
>> +}
>> +
>> +/*
>> + * This subtype enable to set the ABI, which ensure that the eBPF context and
>> + * program behavior will be compatible with this Landlock rule.
>> + */
>> +SEC("subtype")
>> +static const union bpf_prog_subtype _subtype = {
>> +	.landlock_rule = {
>> +		.abi = 1,
>> +		.event = LANDLOCK_SUBTYPE_EVENT_FS,
>> +		.ability = LANDLOCK_SUBTYPE_ABILITY_DEBUG,
>> +	}
>> +};
> 
> from rule writer perspective can you somehow merge subtype definition
> with the program? It seems they go hand in hand.
> Like section name of the program can be:
> SEC("landlock_rule1/event=fs/ability=debug")
> static int landlock_fs_prog1(struct landlock_context *ctx)...
> and the loader can parse this string and prepare appropriate
> data structures for the kernel.

Right, I'll try that.


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