Re: [PATCH] sysctl: fix proc_dobool() usability

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 10, 2023 at 03:58:23PM +0100, Ondrej Mosnacek wrote:
> Currently proc_dobool expects a (bool *) in table->data, but sizeof(int)
> in table->maxsize, because it uses do_proc_dointvec() directly.
> 
> This is unsafe for at least two reasons:
> 1. A sysctl table definition may use { .data = &variable, .maxsize =
>    sizeof(variable) }, not realizing that this makes the sysctl unusable
>    (see the Fixes: tag) and that they need to use the completely
>    counterintuitive sizeof(int) instead.
> 2. proc_dobool() will currently try to parse an array of values if given
>    .maxsize >= 2*sizeof(int), but will try to write values of type bool
>    by offsets of sizeof(int), so it will not work correctly with neither
>    an (int *) nor a (bool *). There is no .maxsize validation to prevent
>    this.
> 
> Fix this by:
> 1. Constraining proc_dobool() to allow only one value and .maxsize ==
>    sizeof(bool).
> 2. Wrapping the original struct ctl_table in a temporary one with .data
>    pointing to a local int variable and .maxsize set to sizeof(int) and
>    passing this one to proc_dointvec(), converting the value to/from
>    bool as needed (using proc_dou8vec_minmax() as an example).
> 3. Extending sysctl_check_table() to enforce proc_dobool() expectations.
> 4. Fixing the proc_dobool() docstring (it was just copy-pasted from
>    proc_douintvec, apparently...).
> 5. Converting all existing proc_dobool() users to set .maxsize to
>    sizeof(bool) instead of sizeof(int).
> 
> Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
> Fixes: a2071573d634 ("sysctl: introduce new proc handler proc_dobool")
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>

Ah nice, thanks for tracking this down.

Acked-by: Kees Cook <keescook@chromium.org>

I've long wanted to replace all the open-coded sysctl initializers with
a helper macro so it's hard to make mistakes. e.g.

#define _SYSCTL_HANDLER(var)					\
		_Generic((var),					\
			default:	proc_dointvec_minmax,	\
			unsigned int:	proc_douintvec_minmax,	\
			unsigned long:	proc_doulongvec_minmax,	\
			char:		proc_dou8vec_minmax,	\
			bool:		proc_dobool,		\
			char *:		proc_dostring)

#define _SYSCTL_MINVAL(var)				\
		_Generic((var),				\
			default:	0,		\
			int:		INT_MIN,	\
			unsigned int:	UINT_MIN,	\
			unsigned long:	ULONG_MIN,	\
			char:		U8_MIN,		\
			bool:		0)

#define _SYSCTL_MAXVAL(var)				\
		_Generic((var),				\
			default:	0,		\
			int:		INT_MAX,	\
			unsigned int:	UINT_MAX,	\
			unsigned long:	ULONG_MAX,	\
			char:		U8_MAX,		\
			bool:		1)

#define _SYSCTL_VAR(_name, _var, _mode, _handler, _len, _min, _max)	\
	{								\
		.procname	= _name,				\
		.data		= &(_var),				\
		.maxlen		= _len,					\
		.mode		= _mode,				\
		.proc_handler	= _handler,				\
		.minlen		= _min,					\
		.maxlen		= _min,					\
	}

/* single value */
#define SYSCTL_VAL(var)							\
	_SYSCTL_VAR(#var, var, 0644, _SYSCTL_HANDLER(var), sizeof(var),	\
		    _SYSCTL_MINVAL(var), _SYSCTL_MAXVAL(var))

/* single value with min/max */
#define SYSCTL_VAL_MINMAX(_var, _min, _max)				\
	_SYSCTL_VAR(#var, var, 0644, _SYSCTL_HANDLER(var), sizeof(var),	\
		    _min, _max)

/* Strings... */
#define SYSCTL_STR(_var, _len)						\
	_SYSCTL_VAR(#var, var, 0644, _SYSCTL_HANDLER(var), _len, 0, 0)

/* Arrays... */
#define SYSCTL_ARRAY(_var)						\
	_SYSCTL_VAR(#var, var, 0644, _SYSCTL_HANDLER(var),		\
		    ARRAY_SIZE(_var), _SYSCTL_MINVAL(var),		\
		    _SYSCTL_MAXVAL(var)

Totally untested. I think this would cover the vast majority of sysctl
initializers.

-- 
Kees Cook