linux-kernel - Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

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Date:   Thu, 22 Jun 2023 19:21:03 +0100
From:   Matthew Wilcox <willy@...radead.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>,
        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,
        david@...hat.com, debug@...osinc.com, szabolcs.nagy@....com,
        torvalds@...ux-foundation.org, broonie@...nel.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

On Mon, Jun 12, 2023 at 05:10:42PM -0700, Rick Edgecombe wrote:
> +++ b/include/linux/mm.h
> @@ -342,7 +342,36 @@ extern unsigned int kobjsize(const void *objp);
>  #endif /* CONFIG_ARCH_HAS_PKEYS */
>  
>  #ifdef CONFIG_X86_USER_SHADOW_STACK
> -# define VM_SHADOW_STACK	VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
> +/*
> + * This flag should not be set with VM_SHARED because of lack of support
> + * core mm. It will also get a guard page. This helps userspace protect
> + * itself from attacks. The reasoning is as follows:
> + *
> + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
> + * INCSSP instruction can increment the shadow stack pointer. It is the
> + * shadow stack analog of an instruction like:
> + *
> + *   addq $0x80, %rsp
> + *
> + * However, there is one important difference between an ADD on %rsp
> + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the
> + * memory of the first and last elements that were "popped". It can be
> + * thought of as acting like this:
> + *
> + * READ_ONCE(ssp);       // read+discard top element on stack
> + * ssp += nr_to_pop * 8; // move the shadow stack
> + * READ_ONCE(ssp-8);     // read+discard last popped stack element
> + *
> + * The maximum distance INCSSP can move the SSP is 2040 bytes, before
> + * it would read the memory. Therefore a single page gap will be enough
> + * to prevent any operation from shifting the SSP to an adjacent stack,
> + * since it would have to land in the gap at least once, causing a
> + * fault.
> + *
> + * Prevent using INCSSP to move the SSP between shadow stacks by
> + * having a PAGE_SIZE guard gap.
> + */
> +# define VM_SHADOW_STACK	VM_HIGH_ARCH_5
>  #else
>  # define VM_SHADOW_STACK	VM_NONE
>  #endif

This is a lot of very x86-specific language in a generic header file.
I'm sure there's a better place for all this text.