Re: [stable:PATCH] KVM: arm64: Reset PMC_EL0 to avoid a panic() on systems with no PMU

From: Greg KH <gregkh@linuxfoundation.org>
To: James Morse <james.morse@arm.com>
Cc: Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.cs.columbia.edu, stable@vger.kernel.org
Subject: Re: [stable:PATCH] KVM: arm64: Reset PMC_EL0 to avoid a panic() on systems with no PMU
Date: Sat, 12 Mar 2022 12:09:33 +0100	[thread overview]
Message-ID: <Yix/bcdSsMGvY/aw@kroah.com> (raw)
In-Reply-To: <20220308162939.603335-1-james.morse@arm.com>

On Tue, Mar 08, 2022 at 04:29:39PM +0000, James Morse wrote:
> The logic in commit 2a5f1b67ec57 "KVM: arm64: Don't access PMCR_EL0 when no
> PMU is available" relies on an empty reset handler being benign.  This was
> not the case in earlier kernel versions, so the stable backport of this
> patch is causing problems.
> 
> KVMs behaviour in this area changed over time. In particular, prior to commit
> 03fdfb269009 ("KVM: arm64: Don't write junk to sysregs on reset"), an empty
> reset handler will trigger a warning, as the guest registers have been
> poisoned.
> Prior to commit 20589c8cc47d ("arm/arm64: KVM: Don't panic on failure to
> properly reset system registers"), this warning was a panic().
> 
> Instead of reverting the backport, make it write 0 to the sys_reg[] array.
> This keeps the reset logic happy, and the dodgy value can't be seen by
> the guest as it can't request the emulation.
> 
> The original bug was accessing the PMCR_EL0 register on CPUs that don't
> implement that feature. There is no known silicon that does this, but
> v4.9's ACPI support is unable to find the PMU, so triggers this code:
> 
> | Kernel panic - not syncing: Didn't reset vcpu_sys_reg(24)
> | CPU: 1 PID: 3055 Comm: lkvm Not tainted 4.9.302-00032-g64e078a56789 #13476
> | Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno Development Platform, BIOS EDK II Jul 30 2018
> | Call trace:
> | [<ffff00000808b4b0>] dump_backtrace+0x0/0x1a0
> | [<ffff00000808b664>] show_stack+0x14/0x20
> | [<ffff0000088f0e18>] dump_stack+0x98/0xb8
> | [<ffff0000088eef08>] panic+0x118/0x274
> | [<ffff0000080b50e0>] access_actlr+0x0/0x20
> | [<ffff0000080b2620>] kvm_reset_vcpu+0x5c/0xac
> | [<ffff0000080ac688>] kvm_arch_vcpu_ioctl+0x3e4/0x490
> | [<ffff0000080a382c>] kvm_vcpu_ioctl+0x5b8/0x720
> | [<ffff000008201e44>] do_vfs_ioctl+0x2f4/0x884
> | [<ffff00000820244c>] SyS_ioctl+0x78/0x9c
> | [<ffff000008083a9c>] __sys_trace_return+0x0/0x4
> 
> Cc: <stable@vger.kernel.org> # < v5.3 with 2a5f1b67ec57 backported
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Now queued up for 4.9, 4.14, and 4.19 kernels, thanks!

greg k-h
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