linux-kernel - [PATCH 0/2] KVM: Fix a guest

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Message-ID: <20251202020334.1171351-1-seanjc@google.com>
Date: Mon,  1 Dec 2025 18:03:32 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alexander Potapenko <glider@...gle.com>, Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF

Fix a UAF due to leaving a dangling guest_memfd memslot binding by
disallowing clearing KVM_MEM_GUEST_MEMFD on a memslot.  The intent was
that guest_memfd memslots would be immutable (could only be deleted),
but somewhat ironically we missed the case where KVM_MEM_GUEST_MEMFD
itself is the only flag that's toggled.

This is an ABI change, but I can't imagine anyone was relying on
disappearing a guest_memfd memslot.

Patch 2 hardens against the UAF, and prepares for allowing FLAGS_ONLY
changes on guest_memfd memslots.  Sooner or later, we're going to allow
dirty logging on guest_memfd, so I think it makes sense to guard against
that so that whoever adds dirty logging support doesn't forget to unbind
on a FLAGS_ONLY change.

I'll respond with the syzkaller reproducer (it's comically simple).

Sean Christopherson (2):
  KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot
  KVM: Harden and prepare for modifying existing guest_memfd memslots

 virt/kvm/kvm_main.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

base-commit: 115d5de2eef32ac5cd488404b44b38789362dbe6
-- 
2.52.0.107.ga0afd4fd5b-goog