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

[Dan Williams]

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@xxxxxxxx>
> ---
> 
> 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.

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 ?