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

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Message-ID: <327a23d5-d5c4-4227-aafb-9d4ddd90289e@suse.com>
Date: Mon, 24 Mar 2025 11:59:19 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Dan Williams <dan.j.williams@...el.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 18.03.25 г. 21:06 ч., Dan Williams wrote:
> Nikolay Borisov wrote:
>> If a piece of memory is read from /dev/mem that falls outside of the
>> System Ram region i.e bios data region the kernel creates a shared
>> mapping via xlate_dev_mem_ptr() (this behavior was introduced by
>> 9aa6ea69852c ("x86/tdx: Make pages shared in ioremap()"). This results
>> in a region having both a shared and a private mapping.
>>
>> Subsequent accesses to this region via the private mapping induce a
>> SEPT violation and a crash of the VMM. In this particular case the
>> scenario was a userspace process reading something from the bios data
>> area at address 0x497 which creates a shared mapping, and a followup
>> reboot accessing __va(0x472) which access pfn 0 via the private mapping
>> causing mayhem.
>>
>> Fix this by simply forbidding access to /dev/mem when running as an TDX
>> guest.
>>
>> Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
>> ---
>>
>> Sending this now to hopefully spur up discussion as to how to handle the described
>> scenario. This was hit on the GCP cloud and was causing their hypervisor to crash.
>>
>> I guess the most pressing question is what will be the most sensible approach to
>> eliminate such situations happening in the future:
>>
>> 1. Should we forbid getting a descriptor to /dev/mem (this patch)
>> 2. Skip creating /dev/mem altogether3
>> 3. Possibly tinker with internals of ioremap to ensure that no memory which is
>> backed by kvm memslots is remapped as shared.
> 
> It seems unfortunate that the kernel is allowing conflicting mappings of
> the same pfn. Is this not just a track_pfn_remap() bug report? In other
> words, whatever established the conflicting private mapping failed to do
> a memtype_reserve() with the encryption setting such that
> track_pfn_remap() could find it and enforce a consistent mapping.

I'm not an expert into this, but looking at the code it seems 
memtype_reserve deals with the memory type w.r.t PAT/MTRR i.e the 
cacheability of the memory, not whether the mapping is consistent w.r.t 
to other, arbitrary attributes.

> 
> Otherwise, kernel_lockdown also disables useful mechanisms like debugfs,
> and feels like it does not solve the underlying problem. Not all
> ioremap() callers in the kernel are aware of a potential
> ioremap_encrypted() dependendency.
> 
>> 4. Eliminate the access to 0x472 from the x86 reboot path, after all we don't
>> really have a proper bios at that address.
>> 5. Something else ?