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Date: Thu, 9 Mar 2017 15:21:45 +0000
From: Mark Rutland <mark.rutland@....com>
To: Russell King - ARM Linux <linux@...linux.org.uk>
Cc: Thomas Garnier <thgarnie@...gle.com>,
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Subject: Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall
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.
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