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Date:   Sun, 28 Apr 2019 08:45:05 +0300
From:   Mike Rapoport <>
To:     Peter Zijlstra <>
        Alexandre Chartre <>,
        Andy Lutomirski <>,
        Borislav Petkov <>,
        Dave Hansen <>,
        "H. Peter Anvin" <>, Ingo Molnar <>,
        James Bottomley <>,
        Jonathan Adams <>,
        Kees Cook <>,
        Paul Turner <>,
        Thomas Gleixner <>,,,
Subject: Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call

On Fri, Apr 26, 2019 at 09:49:56AM +0200, Peter Zijlstra wrote:
> On Fri, Apr 26, 2019 at 12:45:49AM +0300, Mike Rapoport wrote:
> > The initial SCI implementation allows access to any kernel data, but it
> > limits access to the code in the following way:
> > * calls and jumps to known code symbols without offset are allowed
> > * calls and jumps into a known symbol with offset are allowed only if that
> > symbol was already accessed and the offset is in the next page
> > * all other code access are blocked
> So if you have a large function and an in-function jump skips a page
> you're toast.

Right :(
> Why not employ the instruction decoder we have and unconditionally allow
> all direct JMP/CALL but verify indirect JMP/CALL and RET ?

Apparently I didn't dig deep enough to find the instruction decoder :)
Surely I can use it.

> Anyway, I'm fearing the overhead of this one, this cannot be fast.

Well, I think that the verification itself is not what will slow things
down the most. IMHO, the major overhead is coming from cr3 switch.

Sincerely yours,

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