linux-kernel - Re: [PATCH v5 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE

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Message-ID: <202301191658.A599D620B@keescook>
Date:   Thu, 19 Jan 2023 16:58:53 -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>
Subject: Re: [PATCH v5 12/39] x86/mm: Update ptep_set_wrprotect() and
 pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

On Thu, Jan 19, 2023 at 01:22:50PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> When shadow stack is in use, Write=0,Dirty=1 PTE are preserved for
> shadow stack. Copy-on-write PTEs then have Write=0,Cow=1.
> 
> When a PTE goes from Write=1,Dirty=1 to Write=0,Cow=1, it could
> become a transient shadow stack PTE in two cases:
> 
> 1. Some processors can start a write but end up seeing a Write=0 PTE by
>    the time they get to the Dirty bit, creating a transient shadow stack
>    PTE. However, this will not occur on processors supporting shadow
>    stack, and a TLB flush is not necessary.
> 
> 2. When _PAGE_DIRTY is replaced with _PAGE_COW non-atomically, a transient
>    shadow stack PTE can be created as a result. Thus, prevent that with
>    cmpxchg.
> 
> In the case of pmdp_set_wrprotect(), for nopmd configs the ->pmd operated
> on does not exist and the logic would need to be different. Although the
> extra functionality will normally be optimized out when user shadow
> stacks are not configured, also exclude it in the preprocessor stage so
> that it will still compile. User shadow stack is not supported there by
> Linux anyway. Leave the cpu_feature_enabled() check so that the
> functionality also gets disabled based on runtime detection of the
> feature.
> 
> Similarly, compile it out in ptep_set_wrprotect() due to a clang warning
> on i386. Like above, the code path should get optimized out on i386
> since shadow stack is not supported on 32 bit kernels, but this makes
> the compiler happy.
> 
> Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many
> insights to the issue. Jann Horn provided the cmpxchg solution.
> 
> 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>

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

-- 
Kees Cook