On Mon, Apr 24, 2017 at 11:23 PM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Kees Cook <keesc...@chromium.org> wrote:
>
>> On Mon, Apr 10, 2017 at 9:44 AM, Thomas Garnier <thgar...@google.com> 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
>> >
>> > The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
>> > added so each architecture can optimize this change.
>> >
>> > Signed-off-by: Thomas Garnier <thgar...@google.com>
>> > Tested-by: Kees Cook <keesc...@chromium.org>
>>
>> Ingo, I think this series is ready. Can you pull it? (And if not, what
>> should next steps be?)
>
> I have some feedback for other patches in this series, plus for this one as
> well:
>
>> > +/*
>> > + * 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 verify_pre_usermode_state(void) {
>> > + BUG_ON(!segment_eq(get_fs(), USER_DS));
>> > +}
>
> That's not standard kernel coding style.
>
> Also, patch titles should start with a verb - 75% of the series doesn't.
Will fix both.
>
>> > +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>> > +#define __CHECK_USER_CALLER() \
>> > + bool user_caller = segment_eq(get_fs(), USER_DS)
>> > +#define __VERIFY_PRE_USERMODE_STATE() \
>> > + if (user_caller) verify_pre_usermode_state()
>> > +#else
>> > +#define __CHECK_USER_CALLER()
>> > +#define __VERIFY_PRE_USERMODE_STATE()
>> > +asmlinkage void address_limit_check_failed(void);
>> > +#endif
>
>> > +#ifdef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>
> That Kconfig name is way too long.
>
> Plus please don't put logical operations into Kconfig names.
>
>> > +/*
>> > + * This function is called when an architecture specific implementation
>> > detected
>> > + * an invalid address limit. The generic user-mode state checker will
>> > finish on
>> > + * the appropriate BUG_ON.
>> > + */
>> > +asmlinkage void address_limit_check_failed(void)
>> > +{
>> > + verify_pre_usermode_state();
>> > + panic("address_limit_check_failed called with a valid user-mode
>> > state");
>> > +}
>> > +#endif
>
> Awful naming all around:
>
> verify_pre_usermode_state()
> address_limit_check_failed()
>
> Both names start with very common names that makes one read these again and
> again.
> (And yes, there's lots of bad names in the kernel, but we should not follow
> bad
> examples.)
>
> Best practice for such functionality is to use a common prefix that is both
> easy
> to recognize and easy to skip. For example we could use 'addr_limit_check' as
> the
> prefix:
>
> addr_limit_check_failed()
> addr_limit_check_syscall()
>
> No need to over-specify it that it's a "pre" check - it's obvious from
> existing
> implementation and should be documented in the function itself for new
> implementations.
>
> Harmonize the Kconfig namespace to the common prefix as well, i.e. use
> something
> like:
>
> CONFIG_ADDR_LIMIT_CHECK
>
> No need to add 'ARCH' I think - an architecture that enables this should get
> it
> unconditionally.
>
> etc.
>
> It's all cobbled together I'm afraid and will need more iterations.
Make sense. Thanks for the feedback.
>
> Thanks,
>
> Ingo
--
Thomas
