linux-kernel - Re: [PATCH] x86/mm: Set NX bit when making pages present

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Date:   Mon, 19 Sep 2022 18:14:48 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "Liu, Yujie" <yujie.liu@...el.com>, "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH] x86/mm: Set NX bit when making pages present

On Fri, 2022-09-09 at 08:27 -0700, Dave Hansen wrote:
> 0day ran across a case triggered by module unloading, but that looks
> to be a generic problem.  It presumably goes like this:
> 
>         1. Load module with direct map, P=1,W=1,NX=1
>         2. Map module executable, set P=1,W=0,NX=0
>         3. Free module, land in vfree()->vm_remove_mappings()
>         4. Set P=0 during alias processing, P=0,W=0,NX=0
>         5. Restore kernel mapping via
> set_direct_map_default_noflush(),
>            set P=1,W=1, resulting in P=1,W=1,NX=0

I don't think this is right. CPA skips NX modification on the alias, so
the earlier module CPA's (1-2) shouldn't have touched NX where
set_direct_map() is operating. So NX *shouldn't* have been cleared in
this case.

Clearly somehow it is though. The original report has this in the log:
Notice: NX (Execute Disable) protection cannot be enabled: non-PAE
kernel!

So probably the W^X checker needs to check this non-PAE case
differently.