linux-kernel - Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call isolation

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Date:   Fri, 26 Apr 2019 09:49:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Jonathan Adams <jwadams@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org, x86@...nel.org
Subject: Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call
 isolation

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.

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

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