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

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Mar 09, 2017 at 01:44:56PM +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 09, 2017 at 12:09:55PM +0000, Mark Rutland wrote:
> > On Wed, Mar 08, 2017 at 05:24:53PM -0800, Thomas Garnier wrote:

> > 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
> 
> That's utterly pointless - you've missed a detail.
> 
> > > @@ -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 ... 
> 
> Look at this closely.  has_user_ds() is called _before_ the syscall code
> is invoked.  It's checking what conditions the syscall was entered from.
> If the syscall was entered with the user segment selected, then we run
> a check on the system state _after_ the syscall code has returned.

Indeed; I clearly did not consider this correctly. 

Sorry for the noise.

Thanks,
Mark.