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

Thomas Garnier
2017-04-27T14:16:41Z

On Wed, Apr 26, 2017 at 11:49 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * 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.

I think this construct makes more sense because the first macro check
if the syscall was called by user-mode. I will send an update for this
on this thread.

>
> 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.

I don't think architectures need to do that. The optimizations are
embedding the checks on their architecture-specific code to make it
faster and remove the size impact. The pre/post is fine for the rest.

>
> Thanks,
>
> Ingo

--
Thomas