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Date:   Mon, 3 Oct 2022 11:39:10 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
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        jamorris@linux.microsoft.com, dethoma@microsoft.com
Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory
Message-ID: <202210031134.B0B6B37@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-23-rick.p.edgecombe@intel.com>
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In-Reply-To: <20220929222936.14584-23-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
> [...]
> Still allow FOLL_FORCE to write through shadow stack protections, as it
> does for read-only protections.

As I asked in the cover letter: why do we need to add this for shstk? It
was a mistake for general memory. :P

> [...]
> diff --git a/mm/gup.c b/mm/gup.c
> index 5abdaf487460..56da98f3335c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1043,7 +1043,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>  		return -EFAULT;
>  
>  	if (write) {
> -		if (!(vm_flags & VM_WRITE)) {
> +		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
>  			if (!(gup_flags & FOLL_FORCE))
>  				return -EFAULT;
>  			/*

How about this instead:

  		return -EFAULT;
  
 	if (write) {
+		if (vm_flags & VM_SHADOW_STACK)
+			return -EFAULT;
 		if (!(vm_flags & VM_WRITE)) {
 			if (!(gup_flags & FOLL_FORCE))
 				return -EFAULT;


-- 
Kees Cook