Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall

[Thomas Garnier <thgarnie@google.com>]

On Thu, Mar 9, 2017 at 4:09 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> Hi,
>
> On Wed, Mar 08, 2017 at 05:24:53PM -0800, Thomas Garnier wrote:
>> This patch ensures a syscall does not return to user-mode with a kernel
>> address limit. If that happened, a process can corrupt kernel-mode
>> memory and elevate privileges.
>>
>> For example, it would mitigation this bug:
>>
>> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>>
>> If the CONFIG_BUG_ON_DATA_CORRUPTION option is enabled, an incorrect
>> state will result in a BUG_ON.
>>
>> The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
>> added so each architecture can optimize this change.
>
>> +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>> +static inline bool has_user_ds(void) {
>> +     bool ret = segment_eq(get_fs(), USER_DS);
>> +     // Prevent re-ordering the call
>> +     barrier();
>
> What ordering are we trying to ensure, that isn't otherwise given?
>
> We expect get_fs() and set_fs() to be ordered w.r.t. each other and
> w.r.t. uaccess uses, or we'd need barriers all over the place.
>
> Given that, I can't see why we need a barrier here. So this needs a
> better comment, at least.
>

I was half sure of that so that's why I added the barrier. If it is
not needed then I can remove it. Thanks!

>> +     return ret;
>> +}
>> +#else
>> +static inline bool has_user_ds(void) {
>> +     return false;
>> +}
>> +#endif
>
> It would be simpler to wrap the call entirely, e.g. have:
>
> #ifdef CONFIG_WHATEVER
> static inline void verify_pre_usermode_state(void)
> {
>         if (segment_eq(get_fs(), USER_DS))
>                 __verify_pre_usermode_state();
> }
> #else
> static inline void verify_pre_usermode_state(void) { }
> #endif
>
>> @@ -199,7 +215,10 @@ extern struct trace_event_functions exit_syscall_print_funcs;
>>       asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));      \
>>       asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))       \
>>       {                                                               \
>> +             bool user_caller = has_user_ds();                       \
>>               long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));  \
>> +             if (user_caller)                                        \
>> +                     verify_pre_usermode_state();                    \
>
> ... then we can unconditionally use verify_pre_usermode_state() here ...

Not sure I understood that point. The goal is to see if get_fs was
changed, that's why I check before the syscall and I want to ensure
the call is not shuffled after the syscall, therefore the original
barrier.

>
>>               __MAP(x,__SC_TEST,__VA_ARGS__);                         \
>>               __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));       \
>>               return ret;                                             \
>
> [...]
>
>> +/* Called before coming back to user-mode */
>> +asmlinkage void verify_pre_usermode_state(void)
>
> ... and we just prepend a couple of underscores here.
>
>> +{
>> +     if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
>> +                               "incorrect get_fs() on user-mode return"))
>> +             set_fs(USER_DS);
>> +}
>
> Thanks,
> Mark.



-- 
Thomas