Re: [patch V3 01/13] entry: Provide generic syscall entry functionality

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> On Sat, Jul 18, 2020 at 7:16 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>> > FWIW, TIF_USER_RETURN_NOTIFY is a bit of an odd duck: it's an
>> > entry/exit word *and* a context switch word.  The latter is because
>> > it's logically a per-cpu flag, not a per-task flag, and the context
>> > switch code moves it around so it's always set on the running task.
>>
>> Gah, I missed the context switch thing of that. That stuff is hideous.
>
> It's also delightful because anything that screws up that dance (such
> as failure to do the exit-to-usermode path exactly right) likely
> results in an insta-root-hole.  If we fail to run user return
> notifiers, we can run user code with incorrect syscall MSRs, etc.

Looking at it deeper, having that thing in the loop is a pointless
exercise. This really wants to be done _after_ the loop.

Thanks,

        tglx