FAILED: patch "[PATCH] xen/pcifront: check that an AER callback exists before calling it" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 3b4854b69ee70fffb2fa7acac09e715a7efcbae1
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060339-failed-5.15-3b4854b69ee7@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 3b4854b69ee70fffb2fa7acac09e715a7efcbae1 Mon Sep 17 00:00:00 2001
From: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Date: Tue, 6 Oct 2026 11:34:43 +0900
Subject: [PATCH] xen/pcifront: check that an AER callback exists before
 calling it

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>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20261006023443.338194-1-yhlee@isslab.korea.ac.kr>
---
 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 0dea8a69fa36c..b8f2921cc253f 100644
--- a/drivers/pci/xen-pcifront.c
+++ b/drivers/pci/xen-pcifront.c
@@ -598,11 +598,16 @@ 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,
-- 
2.53.0