Re: [PATCH Part1 RFC v3 14/22] x86/mm: Add support to validate memory when changing C-bit

From: Borislav Petkov <bp@alien8.de>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, linux-efi@vger.kernel.org,
	platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev,
	linux-mm@kvack.org, linux-crypto@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	tony.luck@intel.com, npmccallum@redhat.com
Subject: Re: [PATCH Part1 RFC v3 14/22] x86/mm: Add support to validate memory when changing C-bit
Date: Mon, 14 Jun 2021 21:27:50 +0200	[thread overview]
Message-ID: <YMetti2R6SbC0mI0@zn.tnic> (raw)
In-Reply-To: <267dc549-fcef-480e-891c-effd3d5b2058@amd.com>

On Mon, Jun 14, 2021 at 08:05:51AM -0500, Brijesh Singh wrote:
> Guest OS depend on the hypervisor to assist in this operation. The loop
> will terminate only after the hypervisor completes the requested
> operation. Guest is not protecting itself from DoS type of attack. A
> guest should not proceed until hypervisor performs the request page
> state change in the RMP table.

Some of that could be in a comment over that loop, so that it is clear
what the guest strategy is.

> Let me understand, are you saying that hypervisor could trick us into
> believing that page state change completed without actually changing it ?

Nah, I'm just saying that you should verify those ->cur_entry and
->end_entry values.

Of course the guest doesn't protect itself against DoS types of attacks
but this function page_state_vmgexit() here could save ->cur_entry
and ->end_entry on function entry and then compare it each time the
hypercall returns to make sure HV is not doing some shenanigans with
the entries range or even has a bug or so. I.e., it has not changed
->end_entry or ->cur_entry is not going backwards into the buffer.

I know, if uncaught here, it probably will explode later but a cheap
sanity check like that doesn't hurt to have just in case.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette