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Date: Thu, 9 Mar 2017 07:54:40 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Mark Rutland <mark.rutland@....com>
Cc: Russell King - ARM Linux <linux@...linux.org.uk>, David Howells <dhowells@...hat.com>,
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Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall
On Thu, Mar 9, 2017 at 7:21 AM, Mark Rutland <mark.rutland@....com> wrote:
> 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.
>
No problem, I missed that reply so discard my question on the email
few seconds ago.
> Thanks,
> Mark.
--
Thomas
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