Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Sender: Guenter Roeck <groeck7@gmail.com>
Message-ID: <1a6e7165-cdae-6c8c-f57d-159fdb68897b@roeck-us.net>
Date:   Wed, 21 Sep 2022 15:59:50 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Content-Language: en-US
To:     Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org, keescook@chromium.org,
        Sean Christopherson <seanjc@google.com>
References: <YwySW3ROc21hN7g9@hirez.programming.kicks-ass.net>
 <20220921200726.GA3094503@roeck-us.net>
 <4dd594ad-490a-68ee-9e32-fbd9f135bd54@intel.com>
From:   Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH v2] x86/mm: Refuse W^X violations
In-Reply-To: <4dd594ad-490a-68ee-9e32-fbd9f135bd54@intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 9/21/22 13:59, Dave Hansen wrote:
> On 9/21/22 13:07, Guenter Roeck wrote:
>> [    2.042861] CPA refuse W^X violation: 8000000000000063 -> 0000000000000063 range: 0x00000000c00a0000 - 0x00000000c00a0fff PFN a0
>> ILLOPC: cbc65efa: 0f 0b
>> [    2.043267] WARNING: CPU: 0 PID: 1 at arch/x86/mm/pat/set_memory.c:600 __change_page_attr_set_clr+0xdca/0xdd0
> ...
>> [    2.050307] ---[ end trace 0000000000000000 ]---
>> [    2.050762] PCI: PCI BIOS area is rw and x. Use pci=nobios if you want it NX.
>> [    2.051115] kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
>> [    2.051115] BUG: unable to handle page fault for address: c00fd2bf
> 
> This _looks_ like it is working as intended.  The PCI BIOS code tried to
> make a RWX page.  The CPA code refused to do it and presumably returned
> an error, leaving a RW page, non-executable page.  The PCI code didn't
> check the set_memory_x() return code and tried to go execute anyway.
> That resulted in the oops.
> 
> I was able to reproduce this pretty easily.  The workaround from dmesg
> is pci=nobios.  That seems to do the trick for me, although that advise
> was sandwiched between a warning and an oops, so not the easiest to find.
> 
> I'm a bit torn what to do on this one.  Breaking the boot is bad, but so
> is leaving RWX memory around.
> 
> Thoughts?

For my part I'll do what the above suggests, ie run tests with PAE enabled
with pci=nobios command line option. AFAICS that hides the problem in my tests.
I am just not sure if that is really appropriate.

Guenter