linux-kernel - Re: [PATCH] crypto: ccp - Clear PSP interrupt status register before calling handler

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Date:   Wed, 29 Mar 2023 18:19:52 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        linux-crypto@...r.kernel.org
Cc:     John Allen <john.allen@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] crypto: ccp - Clear PSP interrupt status register before
 calling handler

On 3/28/23 10:16, Jeremi Piotrowski wrote:
> The PSP IRQ is edge-triggered (MSI or MSI-X) in all cases supported by
> the psp module so clear the interrupt status register early in the
> handler to prevent missed interrupts. sev_irq_handler() calls wake_up()
> on a wait queue, which can result in a new command being submitted from
> a different CPU. This then races with the clearing of isr and can result
> in missed interrupts. A missed interrupt results in a command waiting
> until it times out, which results in the psp being declared dead.
> 
> This is unlikely on bare metal, but has been observed when running
> virtualized. In the cases where this is observed, sev->cmdresp_reg has
> PSP_CMDRESP_RESP set which indicates that the command was processed
> correctly but no interrupt was asserted.
> 
> The full sequence of events looks like this:
> 
> CPU 1: submits SEV cmd #1
> CPU 1: calls wait_event_timeout()
> CPU 0: enters psp_irq_handler()
> CPU 0: calls sev_handler()->wake_up()
> CPU 1: wakes up; finishes processing cmd #1
> CPU 1: submits SEV cmd #2
> CPU 1: calls wait_event_timeout()
> PSP:   finishes processing cmd #2; interrupt status is still set; no interrupt
> CPU 0: clears intsts
> CPU 0: exits psp_irq_handler()
> CPU 1: wait_event_timeout() times out; psp_dead=true
> 
> Fixes: 200664d5237f ("crypto: ccp: Add Secure Encrypted Virtualization (SEV) command support")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>

Acked-by: Tom Lendacky <thomas.lendacky@....com>

> ---
>   drivers/crypto/ccp/psp-dev.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/crypto/ccp/psp-dev.c b/drivers/crypto/ccp/psp-dev.c
> index c9c741ac8442..949a3fa0b94a 100644
> --- a/drivers/crypto/ccp/psp-dev.c
> +++ b/drivers/crypto/ccp/psp-dev.c
> @@ -42,6 +42,9 @@ static irqreturn_t psp_irq_handler(int irq, void *data)
>   	/* Read the interrupt status: */
>   	status = ioread32(psp->io_regs + psp->vdata->intsts_reg);
>   
> +	/* Clear the interrupt status by writing the same value we read. */
> +	iowrite32(status, psp->io_regs + psp->vdata->intsts_reg);
> +
>   	/* invoke subdevice interrupt handlers */
>   	if (status) {
>   		if (psp->sev_irq_handler)
> @@ -51,9 +54,6 @@ static irqreturn_t psp_irq_handler(int irq, void *data)
>   			psp->tee_irq_handler(irq, psp->tee_irq_data, status);
>   	}
>   
> -	/* Clear the interrupt status by writing the same value we read. */
> -	iowrite32(status, psp->io_regs + psp->vdata->intsts_reg);
> -
>   	return IRQ_HANDLED;
>   }
>