linux-kernel - Re: [PATCH v2] x86/sev: Improve handling of writes to intercepted TSC MSRs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aHeyNvzvbgrWAob5@google.com>
Date: Wed, 16 Jul 2025 07:07:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Nikunj A Dadhania <nikunj@....com>, linux-kernel@...r.kernel.org, bp@...en8.de, 
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, 
	dave.hansen@...ux.intel.com, santosh.shukla@....com
Subject: Re: [PATCH v2] x86/sev: Improve handling of writes to intercepted TSC MSRs

On Wed, Jul 16, 2025, Tom Lendacky wrote:
> On 7/16/25 00:53, Nikunj A Dadhania wrote:
> > From: Sean Christopherson <seanjc@...gle.com>
> > 
> > Currently, when a Secure TSC enabled SNP guest attempts to write to the
> > intercepted GUEST_TSC_FREQ MSR (a read-only MSR), the guest kernel response
> > incorrectly implies a VMM configuration error, when in fact it is the usual
> > VMM configuration to intercept writes to read-only MSRs, unless explicitly
> > documented.
> > 
> > Modify the intercepted TSC MSR #VC handling:
> > * Write to GUEST_TSC_FREQ will generate a #GP instead of terminating the
> >   guest
> > * Write to MSR_IA32_TSC will generate a #GP instead of silently ignoring it
> > 
> > Add a WARN_ONCE to log the incident, as well-behaved SNP guest kernels
> > should never attempt to write to these MSRs.
> > 
> > However, continue to terminate the guest when reading from intercepted
> > GUEST_TSC_FREQ MSR with Secure TSC enabled, as intercepted reads indicate
> > an improper VMM configuration for Secure TSC enabled SNP guests.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Feel free to drop me as author and just give me a Reported-by or Suggested-by.
At this point, I ain't doing a whole lot of anything for this patch :-)

> > +	if (WARN_ON_ONCE(write)) {
> 
> Do we want to capture individual WARNs for each MSR? I guess I'm ok with
> a single WARN for either MSR, but just asking the question.

Or don't WARN at all.  If the caller is doing a bare wrmsrq(), then the kernel
will WARN in ex_handler_msr().  If the caller is doing wrmsrq_safe(), do we care
that they're being deliberately weird?