linux-kernel - Re: [PATCH v5 16/39] x86/mm: Check shadow stack page fault errors

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Message-ID: <202301191659.1B1439CF7@keescook>
Date:   Thu, 19 Jan 2023 16:59:53 -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 16/39] x86/mm: Check shadow stack page fault errors

On Thu, Jan 19, 2023 at 01:22:54PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> The CPU performs "shadow stack accesses" when it expects to encounter
> shadow stack mappings. These accesses can be implicit (via CALL/RET
> instructions) or explicit (instructions like WRSS).
> 
> Shadow stack accesses to shadow-stack mappings can result in faults in
> normal, valid operation just like regular accesses to regular mappings.
> Shadow stacks need some of the same features like delayed allocation, swap
> and copy-on-write. The kernel needs to use faults to implement those
> features.
> 
> The architecture has concepts of both shadow stack reads and shadow stack
> writes. Any shadow stack access to non-shadow stack memory will generate
> a fault with the shadow stack error code bit set.
> 
> This means that, unlike normal write protection, the fault handler needs
> to create a type of memory that can be written to (with instructions that
> generate shadow stack writes), even to fulfill a read access. So in the
> case of COW memory, the COW needs to take place even with a shadow stack
> read. Otherwise the page will be left (shadow stack) writable in
> userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE
> for shadow stack accesses, even if the access was a shadow stack read.
> 
> For the purpose of making this clearer, consider the following example.
> If a process has a shadow stack, and forks, the shadow stack PTEs will
> become read-only due to COW. If the CPU in one process performs a shadow
> stack read access to the shadow stack, for example executing a RET and
> causing the CPU to read the shadow stack copy of the return address, then
> in order for the fault to be resolved the PTE will need to be set with
> shadow stack permissions. But then the memory would be changeable from
> userspace (from CALL, RET, WRSS, etc). So this scenario needs to trigger
> COW, otherwise the shared page would be changeable from both processes.
> 
> Shadow stack accesses can also result in errors, such as when a shadow
> stack overflows, or if a shadow stack access occurs to a non-shadow-stack
> mapping. Also, generate the errors for invalid shadow stack accesses.
> 
> 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