linux-kernel - Re: [PATCH v5 07/39] x86: Add user control-protection fault handler

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Message-ID: <202301191649.5283D6C@keescook>
Date:   Thu, 19 Jan 2023 16:50:25 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v5 07/39] x86: Add user control-protection fault handler

On Thu, Jan 19, 2023 at 01:22:45PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> A control-protection fault is triggered when a control-flow transfer
> attempt violates Shadow Stack or Indirect Branch Tracking constraints.
> For example, the return address for a RET instruction differs from the copy
> on the shadow stack.
> 
> There already exists a control-protection fault handler for handling kernel
> IBT faults. Refactor this fault handler into separate user and kernel
> handlers, like the page fault handler. Add a control-protection handler
> for usermode. To avoid ifdeffery, put them both in a new file cet.c, which
> is compiled in the case of either of the two CET features supported in the
> kernel: kernel IBT or user mode shadow stack. Move some static inline
> functions from traps.c into a header so they can be used in cet.c.
> 
> Opportunistically fix a comment in the kernel IBT part of the fault
> handler that is on the end of the line instead of preceding it.
> 
> Keep the same behavior for the kernel side of the fault handler, except for
> converting a BUG to a WARN in the case of a #CP happening when the feature
> is missing. This unifies the behavior with the new shadow stack code, and
> also prevents the kernel from crashing under this situation which is
> potentially recoverable.
> 
> The control-protection fault handler works in a similar way as the general
> protection fault handler. It provides the si_code SEGV_CPERR to the signal
> handler.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

This diff would have been a bit easier to review if the file move was
separate from the addition of the handler, but regardless:

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook