Re: [PATCH v2] xen/pcifront: check that an AER callback exists before calling it

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Oct 06, 2026 at 11:34:43AM +0900, Yehyeong Lee wrote:
> pcifront_common_process() handles an AER request from the backend by
> dispatching on aer_op.cmd to the bound driver's PCI error handler.  It
> only checks that err_handler and err_handler->error_detected are present,
> then for the mmio_enabled, slot_reset and resume commands it calls the
> corresponding callback unconditionally.  Those three callbacks are
> optional -- the PCI core NULL-checks each of them individually before
> use -- and many drivers (for example igb, igc, ice and ixgbevf) install
> error_detected without all of them.
> 
> aer_op.cmd comes from the shared ring, so a malicious or buggy backend
> can send XEN_PCI_OP_aer_mmio (or _slotreset/_resume) for a device whose
> driver leaves that callback NULL and make the frontend call through a
> NULL pointer, crashing the guest.
> 
> Check each callback before calling it.  When mmio_enabled or slot_reset is
> absent, return PCI_ERS_RESULT_RECOVERED so recovery proceeds, as the PCI
> core does for a missing callback; returning PCI_ERS_RESULT_NONE would
> instead make xen-pciback tear the guest domain down.
> 
> Fixes: 956a9202cd12 ("xen-pcifront: Xen PCI frontend driver.")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
> v2: return PCI_ERS_RESULT_RECOVERED for an absent mmio_enabled/slot_reset
>     instead of PCI_ERS_RESULT_NONE, which xen-pciback treats as fatal and
>     uses to tear the domain down.  (Bjorn acked v1; the ack is dropped here
>     because v2 changes the value returned for an absent callback.)
> v1: https://lore.kernel.org/all/20261005133019.284053-1-yhlee@isslab.korea.ac.kr/
> 
> There is an in-flight fix for a refcount leak in this same function
> ("xen/pcifront: Fix PCI device reference leak in AER handling"); this
> change is orthogonal and applies in either order.
> 
>  drivers/pci/xen-pcifront.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/xen-pcifront.c b/drivers/pci/xen-pcifront.c
> index cffc32d660327..5c6f7796d204a 100644
> --- a/drivers/pci/xen-pcifront.c
> +++ b/drivers/pci/xen-pcifront.c
> @@ -599,11 +599,16 @@ static pci_ers_result_t pcifront_common_process(int cmd,
>  		case XEN_PCI_OP_aer_detected:
>  			return pdrv->err_handler->error_detected(pcidev, state);
>  		case XEN_PCI_OP_aer_mmio:
> -			return pdrv->err_handler->mmio_enabled(pcidev);
> +			if (pdrv->err_handler->mmio_enabled)
> +				return pdrv->err_handler->mmio_enabled(pcidev);
> +			return PCI_ERS_RESULT_RECOVERED;
>  		case XEN_PCI_OP_aer_slotreset:
> -			return pdrv->err_handler->slot_reset(pcidev);
> +			if (pdrv->err_handler->slot_reset)
> +				return pdrv->err_handler->slot_reset(pcidev);
> +			return PCI_ERS_RESULT_RECOVERED;
>  		case XEN_PCI_OP_aer_resume:
> -			pdrv->err_handler->resume(pcidev);
> +			if (pdrv->err_handler->resume)
> +				pdrv->err_handler->resume(pcidev);
>  			return PCI_ERS_RESULT_NONE;
>  		default:
>  			dev_err(&pdev->xdev->dev,
> -- 
> 2.43.0
>