linux-kernel - Re: [PATCH v4 16/39] x86/mm: Check Shadow Stack page fault errors

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Message-ID: <Y7WN5WxENBrhkJXU@zn.tnic>
Date:   Wed, 4 Jan 2023 15:32:05 +0100
From:   Borislav Petkov <bp@...en8.de>
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>,
        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,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 16/39] x86/mm: Check Shadow Stack page fault errors

On Fri, Dec 02, 2022 at 04:35:43PM -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 stacks accesses to shadow-stack mappings can see 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.

You lost me here: by "shadow stack access to non-shadow stack memory" you mean
the explicit one using WRU*SS?

> 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.

I guess I'm missing an example here: are we talking here about a user process
getting its shadow stack pages allocated and them being COW first and on the
first shstk operation, it would generate that fault?

> @@ -1331,6 +1345,30 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * When a page becomes COW it changes from a shadow stack permissioned

Unknown word [permissioned] in comment.

> +	 * page (Write=0,Dirty=1) to (Write=0,Dirty=0,CoW=1), which is simply
> +	 * read-only to the CPU. When shadow stack is enabled, a RET would
> +	 * normally pop the shadow stack by reading it with a "shadow stack
> +	 * read" access. However, in the COW case the shadow stack memory does
> +	 * not have shadow stack permissions, it is read-only. So it will
> +	 * generate a fault.
> +	 *
> +	 * For conventionally writable pages, a read can be serviced with a
> +	 * read only PTE, and COW would not have to happen. But for shadow
> +	 * stack, there isn't the concept of read-only shadow stack memory.
> +	 * If it is shadow stack permissioned, it can be modified via CALL and

Ditto.

> +	 * RET instructions. So COW needs to happen before any memory can be
> +	 * mapped with shadow stack permissions.
> +	 *
> +	 * Shadow stack accesses (read or write) need to be serviced with
> +	 * shadow stack permissioned memory, so in the case of a shadow stack

Is this some new formulation I haven't heard about yet?

"Permissioned <something>"?

> +	 * read access, treat it as a WRITE fault so both COW will happen and
> +	 * the write fault path will tickle maybe_mkwrite() and map the memory
> +	 * shadow stack.
> +	 */
> +	if (error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_WRITE)
>  		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_INSTR)
> -- 
> 2.17.1
> 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette