linux-kernel - Re: [PATCH 06/11] x86/fault: Improve kernel-executing-user-memory handling

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Date:   Wed, 3 Feb 2021 17:23:13 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 06/11] x86/fault: Improve kernel-executing-user-memory
 handling

On Sun, Jan 31, 2021 at 09:24:37AM -0800, Andy Lutomirski wrote:
> Right now we treat the case of the kernel trying to execute from user
> memory more or less just like the kernel getting a page fault on a user
> access.  In the failure path, we check for erratum #93, try to otherwise
> fix up the error, and then oops.
> 
> If we manage to jump to the user address space, with or without SMEP, we
> should not try to resolve the page fault.  This is an error, pure and
> simple.  Rearrange the code so that we catch this case early, check for
> erratum #93, and bail out.

And I'm scratching my head why are you talking about kernel trying
to execute from user memory but doh, that erratum truncates the high
32-bits of rIP and the #PF address looks like a user address but
X86_PF_USER is clear.

Aha.

-- 
Regards/Gruss,
    Boris.

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