linux-kernel - Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

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Date:   Mon, 17 Jan 2022 18:45:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mark Rutland <mark.rutland@....com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
        benh@...nel.crashing.org, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        david@...hat.com, frankja@...ux.ibm.com, frederic@...nel.org,
        gor@...ux.ibm.com, hca@...ux.ibm.com, imbrenda@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, nsaenzju@...hat.com, palmer@...belt.com,
        paulmck@...nel.org, paulus@...ba.org, paul.walmsley@...ive.com,
        seanjc@...gle.com, suzuki.poulose@....com, tglx@...utronix.de,
        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

On 1/14/22 16:19, Mark Rutland wrote:
> I also think there is another issue here. When an IRQ is taken from SIE, will
> user_mode(regs) always be false, or could it be true if the guest userspace is
> running? If it can be true I think tha context tracking checks can complain,
> and it*might*  be possible to trigger a panic().

I think that it would be false, because the guest PSW is in the SIE 
block and switched on SIE entry and exit, but I might be incorrect.

Paolo

> In irqentry_enter(), if user_mode(regs) == true, we call
> irqentry_enter_from_user_mode -> __enter_from_user_mode(). There we check that
> the context is CONTEXT_USER, but IIUC that will be CONTEXT_GUEST at this point.
> We also call arch_check_user_regs(), and IIUC this might permit a malicious
> guest to trigger a host panic by way of debug_user_asce(), but I may have
> misunderstood and that might not be possible.