From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794590730; b=woG8sohuP6LovfSzJeA7aouHhsmTYJE56kboK4s64FdHoekpSI4Ojj95lhHcqwNLKMxb MC9QPxL6TyY22FmUByXM4GdLY4FGLmlSpUoBhNH0+F3NNEYt6c1nHGabD0P1L51rHCviY qy3IZZK+pAaGgJUs25vabKhKP2GnPSa+yc33hvsVmEi2rc9iF3wtR5MGEzZajBtgui0sk VBL6O2Y2Tye3YZAKlMIVwyibi6EWnTOgxRS03dUkNA0JJE/oPMv3XDf0F68B0ILTthXZf LhkIRn0qHNoOHQSd8DJNaNyPuFDD8fFntHgyY+xlxl/1QdxcuM0zqWhwiAYDuHLtryw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794590730; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=VRqhLiYHA083I8w4qSXLatZbskX7q7uO1NskiBll704=; b=Sqqf3DDrgdUpUVe7T4lWQjD7S36Yse9jMqBa3p4XnZ8MQZDN8+Y1Fbw0XfWyVho8s3M0 peG4KQ785t6zkkpbBWDgYrAPj3sj6KMEikuc/l1qdEOwaQBC475CQ4XcNMRaJUFJIDjUm ekcSlbkl3MojxSp+an0i23OqD8AfbsblqOYMNBFxDkPTjTrSlVGyRbi/IFMZ0fHPcWybE fuNYvpw79+XnAdgA/gdTx2FXcGWLgff3gQIRtI2oVihJyqNGu19bJDt7grOQvF2NTEOig 8RtHM7WxbmV9eICJPPcoxUf2CNa6lmyhzfSyzp8dRtzFm9ziRbtqQM4Jn1eHSKlMw+g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=ImJrxHwd; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2E2126079D59; Fri, 13 Nov 2026 17:25:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=ImJrxHwd Received: by smtp.kernel.org (Postfix) with ESMTPSA id 24F341F000C6; Fri, 13 Nov 2026 17:25:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794590730; bh=VRqhLiYHA083I8w4qSXLatZbskX7q7uO1NskiBll704=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ImJrxHwdNrhLT5bylFtSH+Fg7WpJTgmnwoCPTohUvCmUP3+ePCW9xSIvHDDN9WGED zd7lfgp7amQt4qqLuACiSYR0B2nXo1Ad1bd2UvWAGuCVfmuXu/CTihfec9DSOt9yi6 GZ3SryYXGrf4ks+aOi+8qhORfqQgyJfuAakF0TkNeu6/SKs2UeV/f7S5O1rvsW43Mz Dt9hYKXQTeLsqzPeZqC+POuqiTPq/HYwmgTwueE8IhCMqa0Syr8OKGmeUP03npQ0BB YzLVM/T1cX7ZfCxwMsTGGa9sPv38d/C7h5VW39OHedaRJMVJOK4qu5u/XFWOtzvL+k ByxaYy/XJso+g== Date: Fri, 13 Nov 2026 17:25:25 +0000 From: "Lorenzo Stoakes (ARM)" To: Sean Christopherson Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Naveen N Rao , Alexandru Elisei Subject: Re: [PATCH v3] KVM: Skip gfn-based MMU notifier invalidations for guest_memfd-only memslots Message-ID: References: <20261113164000.467175-1-seanjc@google.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261113164000.467175-1-seanjc@google.com> On Fri, Nov 13, 2026 at 08:40:00AM -0800, Sean Christopherson wrote: > From: Alexandru Elisei > > Don't invalidate gmem-only memslots when responding to MMU notifier > invalidations, as the entire point of gmem-only memslots is so that KVM > doesn't need a VMA to install mappings into KVM's MMU. Specifically, > page faults are resolved by going straight to guest_memfd, using the gfn > and the associated memslot's gmem.file. I.e. whether or not userspace > happens to have valid mappings to the same gmem instance is irrelevant, > as are any invalidations for those mappings. > > Note! The exception is KVM's gfn_to_pfn_cache implementation which isn't > yet wired up to pull pfns directly from guest_memfd, i.e. still relies on > userspace mappings/VMAs and thus need to be invalidated, e.g. to ensure KVM > doesn't retain a kernel mapping+pointer to PRIVATE memory. However, as of > commit da260bcb56fe ("KVM: pfncache: use a dedicated invalidation sequence > for cache refresh"), GPC invalidation no longer relies on memslot lookups, > because GPCs also support HVA-based mappings. I.e. KVM *must* be able to > detect and handle relevant MMU notifier invalidations even if there is *no* > memslot whatsoever. > > In addition to the obvious performance benefits, avoiding unnecessary and > spurious SPTE zaps and TLB flushes will allow KVM to support features like > ARM's Statistical Profiling Extension (SPE)[1] and x86's Secure AVIC[2], > both of which rely on the pages *never* being unmapped from the stage-2 > page tables. > > Link: https://lore.kernel.org/all/20260903160623.315525-1-alexandru.elisei@arm.com [1] > Link: https://lore.kernel.org/all/cover.1783490022.git.naveen@kernel.org [2] > Cc: Lorenzo Stoakes (ARM) > Cc: Naveen N Rao (AMD) > Signed-off-by: Alexandru Elisei > [sean: rewrite changelog to capture the new gpc behavior] > Signed-off-by: Sean Christopherson Thanks a lot for this Sean + Alex! :) Though, having local AI do a review pass on it flagged an issue (don't worry my own analysis - not pasting some slop here :). I think actually this might need an additional change for x86, since it doesn't have a gmem-only check for the shadow MMU PTE prefetch: direct_map() -> direct_pte_prefetch() -> __direct_pte_prefetch() -> direct_pte_prefetch_many() -> kvm_mmu_prefetch_sptes() -> kvm_prefetch_pages() -> get_user_pages_fast_only() And a VMM that's doing something broken (i.e. trying to take advantage of this as a vulnerability), could then intentionally set up a gmem-only slot to point at ordinary anon memory, munmap(), and have a UAF on the underlying folio due to no MMU notifier invalidation. Fix would be something like: diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index cf4cb586f140..76350dc55d8f 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -3172,6 +3172,9 @@ int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn, unsigned long addr; gfn_t entry = 0; + if (kvm_memslot_is_gmem_only(slot)) + return 0; + addr = gfn_to_hva_many(slot, gfn, &entry); if (kvm_is_error_hva(addr)) return -1; > --- > > v3: v1, but with a different changelog. > > v2: https://lore.kernel.org/all/20260714160411.302386-1-alexandru.elisei@arm.com > > v1: https://lore.kernel.org/all/20260615155244.183044-1-alexandru.elisei@arm.com > > virt/kvm/kvm_main.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index cf4cb586f140..599dea22afb8 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -601,6 +601,10 @@ static __always_inline kvm_mn_ret_t kvm_handle_hva_range(struct kvm *kvm, > unsigned long hva_start, hva_end; > > slot = container_of(node, struct kvm_memory_slot, hva_node[slots->node_idx]); > + > + if (kvm_slot_has_gmem(slot) && kvm_memslot_is_gmem_only(slot)) > + continue; > + > hva_start = max_t(unsigned long, range->start, slot->userspace_addr); > hva_end = min_t(unsigned long, range->end, > slot->userspace_addr + (slot->npages << PAGE_SHIFT)); > > base-commit: d21eef2084a9af87ac2a783bc7c2b175ba114e4e > -- > 2.56.1.703.gf848ed807c-goog > -- Cheers, Lorenzo