[kernel-hardening] Re: [PATCH v8 1/4] syscalls: Verify address limit before returning to user-mode

[Ingo Molnar <mingo@kernel.org>]

* Thomas Garnier <thgarnie@google.com> wrote:

> +
> +/*
> + * Called before coming back to user-mode. Returning to user-mode with an
> + * address limit different than USER_DS can allow to overwrite kernel memory.
> + */
> +static inline void addr_limit_check_syscall(void)
> +{
> +	BUG_ON(!segment_eq(get_fs(), USER_DS));
> +}
> +
> +#ifndef CONFIG_ADDR_LIMIT_CHECK
> +#define __CHECK_USERMODE_SYSCALL() \
> +	bool user_caller = segment_eq(get_fs(), USER_DS)
> +#define __VERIFY_ADDR_LIMIT() \
> +	if (user_caller) addr_limit_check_syscall()
> +#else
> +#define __CHECK_USERMODE_SYSCALL()
> +#define __VERIFY_ADDR_LIMIT()
> +asmlinkage void addr_limit_check_failed(void) __noreturn;
> +#endif

_Please_ harmonize all the externally exposed names and symbols.

There's no reason for this mismash of names:

	CONFIG_ADDR_LIMIT_CHECK

	__CHECK_USERMODE_SYSCALL
	__VERIFY_ADDR_LIMIT

When we could just as easily name them consistently, along the existing pattern:

	CONFIG_ADDR_LIMIT_CHECK

	__SYSCALL_ADDR_LIMIT_CHECK
	__ADDR_LIMIT_CHECK

which should fit into existing nomenclature:

>  #define __SYSCALL_DEFINEx(x, name, ...)					\

But even with that fixed, the whole construct still looks pretty weird:

>  	{								\
> -		long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));	\
> +		long ret;						\
> +		__CHECK_USERMODE_SYSCALL();				\
> +		ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));	\
> +		__ADDR_LIMIT_CHECK();					\
>  		__MAP(x,__SC_TEST,__VA_ARGS__);				\
>  		__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));	\
>  		return ret;						\

I think something like this would be more natural to read:

> +		ADDR_LIMIT_CHECK_PRE();					\
> +		ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));	\
> +		ADDR_LIMIT_CHECK_POST();				\

it's a clear pre/post construct. Also note the lack of double underscores.

BTW., a further simplification would be:

#ifndef ADDR_LIMIT_CHECK_PRE
# define ADDR_LIMIT_CHECK_PRE ...
#endif

This way architectures could override this generic functionality simply by 
defining the helpers. Architectures that don't do that get the generic version. 

Thanks,

	Ingo