Re: [PATCH v6 08/22] bootconfig: init: Allow admin to use bootconfig for init command line

[Kees Cook]

On Sat, Jan 11, 2020 at 01:04:55AM +0900, Masami Hiramatsu wrote:
> Since the current kernel command line is too short to describe
> long and many options for init (e.g. systemd command line options),
> this allows admin to use boot config for init command line.
> 
> All init command line under "init." keywords will be passed to
> init.
> 
> For example,
> 
> init.systemd {
> 	unified_cgroup_hierarchy = 1
> 	debug_shell
> 	default_timeout_start_sec = 60
> }
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  init/main.c |   31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/init/main.c b/init/main.c
> index c0017d9d16e7..dd7da62d99a5 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -139,6 +139,8 @@ char *saved_command_line;
>  static char *static_command_line;
>  /* Untouched extra command line */
>  static char *extra_command_line;
> +/* Extra init arguments */
> +static char *extra_init_args;
>  
>  static char *execute_command;
>  static char *ramdisk_execute_command;
> @@ -372,6 +374,8 @@ static void __init setup_boot_config(void)
>  		pr_info("Load boot config: %d bytes\n", size);
>  		/* keys starting with "kernel." are passed via cmdline */
>  		extra_command_line = xbc_make_cmdline("kernel");
> +		/* Also, "init." keys are init arguments */
> +		extra_init_args = xbc_make_cmdline("init");
>  	}
>  }
>  #else
> @@ -507,16 +511,18 @@ static inline void smp_prepare_cpus(unsigned int maxcpus) { }
>   */
>  static void __init setup_command_line(char *command_line)
>  {
> -	size_t len, xlen = 0;
> +	size_t len, xlen = 0, ilen = 0;
>  
>  	if (extra_command_line)
>  		xlen = strlen(extra_command_line);
> +	if (extra_init_args)
> +		ilen = strlen(extra_init_args) + 4; /* for " -- " */
>  
>  	len = xlen + strlen(boot_command_line) + 1;
>  
> -	saved_command_line = memblock_alloc(len, SMP_CACHE_BYTES);
> +	saved_command_line = memblock_alloc(len + ilen, SMP_CACHE_BYTES);
>  	if (!saved_command_line)
> -		panic("%s: Failed to allocate %zu bytes\n", __func__, len);
> +		panic("%s: Failed to allocate %zu bytes\n", __func__, len + ilen);
>  
>  	static_command_line = memblock_alloc(len, SMP_CACHE_BYTES);
>  	if (!static_command_line)
> @@ -533,6 +539,22 @@ static void __init setup_command_line(char *command_line)
>  	}
>  	strcpy(saved_command_line + xlen, boot_command_line);
>  	strcpy(static_command_line + xlen, command_line);
> +
> +	if (ilen) {
> +		/*
> +		 * Append supplemental init boot args to saved_command_line
> +		 * so that user can check what command line options passed
> +		 * to init.
> +		 */
> +		len = strlen(saved_command_line);
> +		if (!strstr(boot_command_line, " -- ")) {
> +			strcpy(saved_command_line + len, " -- ");
> +			len += 4;
> +		} else
> +			saved_command_line[len++] = ' ';
> +
> +		strcpy(saved_command_line + len, extra_init_args);
> +	}

This isn't safe because it will destroy any argument with " -- " in
quotes and anything after it. For example, booting with:

thing=on acpi_osi="! -- " other=setting

will wreck acpi_osi's value and potentially overwrite "other=settings",
etc.

(Yes, this seems very unlikely, but you can't treat " -- " as special,
the command line string must be correct parsed for double quotes, as
parse_args() does.)

>  }
>  
>  /*
> @@ -759,6 +781,9 @@ asmlinkage __visible void __init start_kernel(void)
>  	if (!IS_ERR_OR_NULL(after_dashes))
>  		parse_args("Setting init args", after_dashes, NULL, 0, -1, -1,
>  			   NULL, set_init_arg);
> +	if (extra_init_args)
> +		parse_args("Setting extra init args", extra_init_args,
> +			   NULL, 0, -1, -1, NULL, set_init_arg);

Here is where you can append the extra_init_args, since parse_args()
will have done the work to find after_dashes correctly.

-Kees

>  
>  	/*
>  	 * These use large bootmem allocations and must precede
> 

-- 
Kees Cook