linux-kernel - Re: [PATCH v6 37/41] selftests/x86: Add shadow stack test

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Message-ID: <bed841c6-5b37-6c91-81c7-5c06788d38c4@redhat.com>
Date:   Tue, 21 Feb 2023 09:48:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>, 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>,
        Kees Cook <keescook@...omium.org>,
        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,
        debug@...osinc.com
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v6 37/41] selftests/x86: Add shadow stack test

On 18.02.23 22:14, Rick Edgecombe wrote:
> Add a simple selftest for exercising some shadow stack behavior:
>   - map_shadow_stack syscall and pivot
>   - Faulting in shadow stack memory
>   - Handling shadow stack violations
>   - GUP of shadow stack memory
>   - mprotect() of shadow stack memory
>   - Userfaultfd on shadow stack memory
> 
> Since this test exercises a recently added syscall manually, it needs
> to find the automatically created __NR_foo defines. Per the selftest
> documentation, KHDR_INCLUDES can be used to help the selftest Makefile's
> find the headers from the kernel source. This way the new selftest can
> be built inside the kernel source tree without installing the headers
> to the system. So also add KHDR_INCLUDES as described in the selftest
> docs, to facilitate this.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> 
> ---


[...]

> +bool gup_write(void *ptr)
> +{
> +	unsigned long val;
> +
> +	lseek(fd, (unsigned long)ptr, SEEK_SET);
> +	if (write(fd, &val, sizeof(val)) < 0)
> +		return 1;

/proc/self/mem is for debug/ptrace access (FOLL_FORCE). I think you 
might also want to add tests for ordinary GUP, checking that we fail to 
obtain a write pin -- and call these tests "gup_ptrace_read" / 
"gup_ptrace_write"

An simple approach would be to trigger a read()/write() on a file opened 
via O_DIRECT, using the shadow stack as buffer. While the write() 
[reading from the page] is expected to work, a read() [writing to the 
page] has to fail.


-- 
Thanks,

David / dhildenb