Re: [PATCH] KVM: x86: Fix a 4.14 backport regression related to userspace/guest FPU

[Sasha Levin <sashal@kernel.org>]

On Mon, Jan 28, 2019 at 12:51:02PM -0800, Sean Christopherson wrote:
>Upstream commit:
>
>    f775b13eedee ("x86,kvm: move qemu/guest FPU switching out to vcpu_run")
>
>introduced a bug, which was later fixed by upstream commit:
>
>    5663d8f9bbe4 ("kvm: x86: fix WARN due to uninitialized guest FPU state")
>
>For reasons unknown, both commits were initially passed-over for
>inclusion in the 4.14 stable branch despite being tagged for stable.
>Eventually, someone noticed that the fixup, commit 5663d8f9bbe4, was
>missing from stable[1], and so it was queued up for 4.14 and included in
>release v4.14.79.
>
>Even later, the original buggy patch, commit f775b13eedee, was also
>applied to the 4.14 stable branch.  Through an unlucky coincidence, the
>incorrect ordering did not generate a conflict between the two patches,
>and led to v4.14.94 and later releases containing a spurious call to
>kvm_load_guest_fpu() in kvm_arch_vcpu_ioctl_run().  As a result, KVM may
>reload stale guest FPU state, e.g. after accepting in INIT event.  This
>can manifest as crashes during boot, segfaults, failed checksums and so
>on and so forth.
>
>Remove the unwanted kvm_{load,put}_guest_fpu() calls, i.e. make
>kvm_arch_vcpu_ioctl_run() look like commit 5663d8f9bbe4 was backported
>after commit f775b13eedee.
>
>[1] https://www.spinics.net/lists/stable/msg263931.html
>
>Fixes: 4124a4cff344 ("x86,kvm: move qemu/guest FPU switching out to vcpu_run")
>Cc: stable@vger.kernel.org
>Cc: Sasha Levin <sashal@kernel.org>
>Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>Cc: Peter Xu <peterx@redhat.com>
>Cc: Rik van Riel <riel@redhat.com>
>Cc: Paolo Bonzini <pbonzini@redhat.com>
>Cc: Radim Krčmář <rkrcmar@redhat.com>
>Reported-by: Roman Mamedov
>Reported-by: Thomas Lindroth <thomas.lindroth@gmail.com>
>Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>

I agree with your analysis and the patch makes sense. Hopefully one of
the KVM folks can Ack.