[PATCH v1] hw/vfio: Fix liveness check in vfio_connect_kvm_msi_virq()

[Tycho Andersen <tycho@kernel.org>]

From: "Tycho Andersen (AMD)" <tycho@kernel.org>

While working on savevm/loadvm for a new vfio device, I encountered the
crash below. Since vfio_connect_kvm_msi_virq() didn't check the ->use flag
for the vector, it would pass an unused vector down to
vfio_cpr_load_vector_fd() which would crash.

Fix this by checking the ->use flag along with the virq number to detect
whether a vector is valid or not.

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555a891ef in vfio_cpr_load_vector_fd (vdev=vdev@entry=0x0,
    name=name@entry=0x555555eeb27e "kvm_interrupt", nr=nr@entry=1) at ../hw/vfio/cpr.c:44
44	    g_autofree char *fdname = STRDUP_VECTOR_FD_NAME(vdev, name);
(gdb) bt
#0  0x0000555555a891ef in vfio_cpr_load_vector_fd
    (vdev=vdev@entry=0x0, name=name@entry=0x555555eeb27e "kvm_interrupt", nr=nr@entry=1)
    at ../hw/vfio/cpr.c:44
#1  0x0000555555ce64a1 in vfio_notifier_init
    (vdev=0x0, e=e@entry=0x5555586971b4, name=name@entry=0x555555eeb27e "kvm_interrupt", nr=nr@entry=1, errp=errp@entry=0x0) at ../hw/vfio/pci.c:79
#2  0x0000555555ce721e in vfio_connect_kvm_msi_virq (vector=0x5555586971a8, nr=nr@entry=1)
    at ../hw/vfio/pci.c:601
#3  0x0000555555cea5a5 in vfio_connect_kvm_msi_virq (nr=1, vector=<optimized out>)
    at ../hw/vfio/pci.c:597
#4  vfio_pci_commit_kvm_msi_virq_batch (vdev=0x55555906de40) at ../hw/vfio/pci.c:822
#5  0x0000555555cea9f2 in vfio_msix_enable (vdev=vdev@entry=0x55555906de40) at ../hw/vfio/pci.c:850
#6  0x0000555555ceb152 in vfio_pci_load_config (vbasedev=0x55555906e900, f=<optimized out>)
    at ../hw/vfio/pci.c:3088
#7  0x0000555555a8c765 in vfio_load_device_config_state (f=0x5555574a43d0, opaque=0x55555906e900)
    at ../hw/vfio/migration.c:278
#8  0x0000555555b3a522 in vmstate_load
    (f=f@entry=0x5555574a43d0, se=se@entry=0x5555591edd40, errp=errp@entry=0x7fffffffe130)
    at ../migration/savevm.c:971
#9  0x0000555555b3ab1a in qemu_loadvm_section_start_full
    (f=f@entry=0x5555574a43d0, type=type@entry=4 '\004', errp=errp@entry=0x7fffffffe130)
    at ../migration/savevm.c:2654
#10 0x0000555555b3e1ee in qemu_loadvm_state_main
    (f=f@entry=0x5555574a43d0, mis=mis@entry=0x5555571de5a0, errp=0x7fffffffe130,
    errp@entry=0x555557157c10 <error_fatal>) at ../migration/savevm.c:2973
#11 0x0000555555b3f7b7 in qemu_loadvm_state
    (f=f@entry=0x5555574a43d0, errp=errp@entry=0x555557157c10 <error_fatal>)
    at ../migration/savevm.c:3058
#12 0x0000555555b40863 in load_snapshot
    (name=0x7fffffffecc9 "foo", vmstate=vmstate@entry=0x0, has_devices=has_devices@entry=false, devices=devices@entry=0x0, errp=errp@entry=0x555557157c10 <error_fatal>) at ../migration/savevm.c:3452
#13 0x0000555555adc211 in qmp_x_exit_preconfig (errp=0x555557157c10 <error_fatal>) at ../system/vl.c:2817
#14 qmp_x_exit_preconfig (errp=0x555557157c10 <error_fatal>) at ../system/vl.c:2802
#15 0x0000555555adf8ed in qemu_init (argc=<optimized out>, argv=<optimized out>) at ../system/vl.c:3849
#16 0x00005555558903fd in main (argc=<optimized out>, argv=<optimized out>) at ../system/main.c:71

Fixes: 30edcb4d4e7a ("vfio-pci: preserve MSI")
Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
---
An alternative implementation of this would be to set ->virq = -1 after
the allocation in vfio_msix_enable(), but since other locations that
checks ->virq >= 0 also check ->use, I added it here too. Maybe it would
be useful to do both.
---
 hw/vfio/pci.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 380dd8c15f..b5280c3d2a 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -589,7 +589,7 @@ static void vfio_connect_kvm_msi_virq(VFIOMSIVector *vector, int nr)
 {
     const char *name = "kvm_interrupt";
 
-    if (vector->virq < 0) {
+    if (!vector->use || vector->virq < 0) {
         return;
     }
 

base-commit: 300438ffbb8d9430cac2fcc15cba6f482b2c0587
-- 
2.55.0