Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 25 Oct 2021 12:59:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Nadav Amit <namit@vmware.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Xu <peterx@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>, x86@kernel.org
Subject: Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault
Message-ID: <YXaOLAIo7B0+1NUc@hirez.programming.kicks-ass.net>
References: <20211021122112.592634-1-namit@vmware.com>
 <20211021122112.592634-4-namit@vmware.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20211021122112.592634-4-namit@vmware.com>
Precedence: bulk

On Thu, Oct 21, 2021 at 05:21:10AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>

> Add a check to prevent access_error() from returning mistakenly that
> page-faults due to instruction fetch are not allowed. Intel SDM does not
> indicate whether "instruction fetch" and "write" in the hardware error
> code are mutual exclusive, so check both before returning whether the
> access is allowed.

Dave, can we get that clarified? It seems a bit naf and leads to
confusing code IMO.

Other than that, the change looks ok to me.

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index b2eefdefc108..e776130473ce 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1100,10 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  				       (error_code & X86_PF_INSTR), foreign))
>  		return 1;
>  
> -	if (error_code & X86_PF_WRITE) {
> +	if (error_code & (X86_PF_WRITE | X86_PF_INSTR)) {
>  		/* write, present and write, not present: */
> -		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> +		if ((error_code & X86_PF_WRITE) &&
> +		    unlikely(!(vma->vm_flags & VM_WRITE)))
>  			return 1;
> +
> +		/* exec, present and exec, not present: */
> +		if ((error_code & X86_PF_INSTR) &&
> +		    unlikely(!(vma->vm_flags & VM_EXEC)))
> +			return 1;
> +
>  		return 0;
>  	}