linux-kernel - Re: [PATCH V2] x86/sev: Mark the code returning to user space as syscall gap

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Message-ID: <Yn5eVA07T4oTkTLl@suse.de>
Date:   Fri, 13 May 2022 15:34:12 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH V2] x86/sev: Mark the code returning to user space as
 syscall gap

Hi,

On Tue, Apr 12, 2022 at 08:49:08PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
> 
> When returning to user space, the %rsp is user controlled value.
> 
> If it is SNP-guest and the hypervisor decides to mess with the code-page
> for this path while a CPU is executing it.  This will cause a #VC on
> that CPU and that could hit in the syscall return path and mislead
> the #VC handler.
> 
> So make ip_within_syscall_gap() return true in this case.

With the SNP guest patches in tip-tree I think it actually becomes
possible that a #VC exception hits in these parts of the execution
stream. It requires good timing by the attacker, but it is not
impossible. Therefore:

Acked-by: Joerg Roedel <jroedel@...e.de>

-- 
Jörg Rödel
jroedel@...e.de

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