[PATCH 6/6] PCI: hv: Fix synchronization between channel callback and hv_compose_msi_msg()

Andrea Parri (Microsoft)
2022-04-07T04:33:01Z

Dexuan wrote:

"[...] when we disable AccelNet, the host PCI VSP driver sends a
PCI_EJECT message first, and the channel callback may set
hpdev->state to hv_pcichild_ejecting on a different CPU. This can
cause hv_compose_msi_msg() to exit from the loop and 'return', and
the on-stack variable 'ctxt' is invalid. Now, if the response
message from the host arrives, the channel callback will try to
access the invalid 'ctxt' variable, and this may cause a crash."

Schematically:

Hyper-V sends PCI_EJECT msg
hv_pci_onchannelcallback()
state = hv_pcichild_ejecting
hv_compose_msi_msg()
alloc and init comp_pkt
state == hv_pcichild_ejecting
Hyper-V sends VM_PKT_COMP msg
hv_pci_onchannelcallback()
retrieve address of comp_pkt
'free' comp_pkt and return
comp_pkt->completion_func()

Dexuan also showed how the crash can be triggered after introducing
suitable delays in the driver code, thus validating the 'assumption'
that the host can still normally respond to the guest's compose_msi
request after the host has started to eject the PCI device.

Fix the synchronization by leveraging the requestor lock as follows:

- Before 'return'-ing in hv_compose_msi_msg(), remove the ID (while
holding the requestor lock) associated to the completion packet.

- Retrieve the address *and call ->completion_func() within a same
(requestor) critical section in hv_pci_onchannelcallback().

Fixes: de0aa7b2f97d3 ("PCI: hv: Fix 2 hang issues in hv_compose_msi_msg()")
Reported-by: Wei Hu <weh@microsoft.com>
Reported-by: Dexuan Cui <decui@microsoft.com>
Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
The "Fixes:" tag is mainly a reference: a back-port would depend
on the entire series (which, in turn, shouldn't be backported to
commits preceding bf5fd8cae3c8f).

drivers/pci/controller/pci-hyperv.c | 33 +++++++++++++++++++++++------
1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index c1322ac37cda9..f1d794f8a5ef1 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1695,7 +1695,7 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
struct pci_create_interrupt3 v3;
} int_pkts;
} __packed ctxt;
-
+ u64 trans_id;
u32 size;
int ret;

@@ -1757,10 +1757,10 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
goto free_int_desc;
}

- ret = vmbus_sendpacket(hpdev->hbus->hdev->channel, &ctxt.int_pkts,
- size, (unsigned long)&ctxt.pci_pkt,
- VM_PKT_DATA_INBAND,
- VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
+ ret = vmbus_sendpacket_getid(hpdev->hbus->hdev->channel, &ctxt.int_pkts,
+ size, (unsigned long)&ctxt.pci_pkt,
+ &trans_id, VM_PKT_DATA_INBAND,
+ VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
if (ret) {
dev_err(&hbus->hdev->device,
"Sending request for interrupt failed: 0x%x",
@@ -1839,6 +1839,15 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)

enable_tasklet:
tasklet_enable(&channel->callback_event);
+ /*
+ * The completion packet on the stack becomes invalid after 'return';
+ * remove the ID from the VMbus requestor if the identifier is still
+ * mapped to/associated with the packet. (The identifier could have
+ * been 're-used', i.e., already removed and (re-)mapped.)
+ *
+ * Cf. hv_pci_onchannelcallback().
+ */
+ vmbus_request_addr_match(channel, trans_id, (unsigned long)&ctxt.pci_pkt);
free_int_desc:
kfree(int_desc);
drop_reference:
@@ -2717,6 +2726,7 @@ static void hv_pci_onchannelcallback(void *context)
struct pci_dev_inval_block *inval;
struct pci_dev_incoming *dev_message;
struct hv_pci_dev *hpdev;
+ unsigned long flags;

buffer = kmalloc(bufferlen, GFP_ATOMIC);
if (!buffer)
@@ -2751,8 +2761,11 @@ static void hv_pci_onchannelcallback(void *context)
switch (desc->type) {
case VM_PKT_COMP:

- req_addr = chan->request_addr_callback(chan, req_id);
+ lock_requestor(chan, flags);
+ req_addr = __vmbus_request_addr_match(chan, req_id,
+ VMBUS_RQST_ADDR_ANY);
if (req_addr == VMBUS_RQST_ERROR) {
+ unlock_requestor(chan, flags);
dev_warn_ratelimited(&hbus->hdev->device,
"Invalid transaction ID %llx\n",
req_id);
@@ -2760,9 +2773,17 @@ static void hv_pci_onchannelcallback(void *context)
}
comp_packet = (struct pci_packet *)req_addr;
response = (struct pci_response *)buffer;
+ /*
+ * Call ->completion_func() within the critical section to make
+ * sure that the packet pointer is still valid during the call:
+ * here 'valid' means that there's a task still waiting for the
+ * completion, and that the packet data is still on the waiting
+ * task's stack. Cf. hv_compose_msi_msg().
+ */
comp_packet->completion_func(comp_packet->compl_ctxt,
response,
bytes_recvd);
+ unlock_requestor(chan, flags);
break;

case VM_PKT_DATA_INBAND:
--
2.25.1