linux-kernel - Re: [RFC PATCH] /dev/mem: Disable /dev/mem under TDX guest

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Message-ID: <babd4e40-0cb6-4796-af86-1944e32f89ee@intel.com>
Date: Tue, 18 Mar 2025 07:48:50 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Nikolay Borisov <nik.borisov@...e.com>, dave.hansen@...ux.intel.com
Cc: kirill.shutemov@...ux.intel.com, linux-coco@...ts.linux.dev,
 x86@...nel.org, linux-kernel@...r.kernel.org, vannapurve@...gle.com
Subject: Re: [RFC PATCH] /dev/mem: Disable /dev/mem under TDX guest

On 3/18/25 04:36, Nikolay Borisov wrote:
> 1. Should we forbid getting a descriptor to /dev/mem (this patch)
> 2. Skip creating /dev/mem altogether3

Like Kirill mentioned, it would be nice to leverage the existing hooks:

        if (!capable(CAP_SYS_RAWIO))
                return -EPERM;

        rc = security_locked_down(LOCKDOWN_DEV_MEM);
        if (rc)
                return rc;

Lockdown seems like a decent fit. We'd also ideally check
lockdown_is_locked_down() in x86 code and spew epithets if someone is
booting a CoCo guest without lockdown.

> 3. Possibly tinker with internals of ioremap to ensure that no memory which is
> backed by kvm memslots is remapped as shared.

It's not just memslots, though. It's any TDX private memory which
includes stuff the TDX module uses like the PAMT or SEPT pages.