xen: properly gate host writes of modified PCI CFG contents

The old logic didn't work as intended when an access spanned multiple
fields (for example a 32-bit access to the location of the MSI Message
Data field with the high 16 bits not being covered by any known field).
Remove it and derive which fields not to write to from the accessed
fields' emulation masks: When they're all ones, there's no point in
doing any host write.

This fixes a secondary issue at once: We obviously shouldn't make any
host write attempt when already the host read failed.

This is XSA-128.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/hw/xen/xen_pt.c b/hw/xen/xen_pt.c
index d095c08..8923582 100644
--- a/hw/xen/xen_pt.c
+++ b/hw/xen/xen_pt.c
@@ -234,7 +234,7 @@ static void xen_pt_pci_write_config(PCIDevice *d, uint32_t addr,
     int index = 0;
     XenPTRegGroup *reg_grp_entry = NULL;
     int rc = 0;
-    uint32_t read_val = 0;
+    uint32_t read_val = 0, wb_mask;
     int emul_len = 0;
     XenPTReg *reg_entry = NULL;
     uint32_t find_addr = addr;
@@ -271,6 +271,9 @@ static void xen_pt_pci_write_config(PCIDevice *d, uint32_t addr,
     if (rc < 0) {
         XEN_PT_ERR(d, "pci_read_block failed. return value: %d.\n", rc);
         memset(&read_val, 0xff, len);
+        wb_mask = 0;
+    } else {
+        wb_mask = 0xFFFFFFFF >> ((4 - len) << 3);
     }
 
     /* pass directly to the real device for passthrough type register group */
@@ -298,6 +301,11 @@ static void xen_pt_pci_write_config(PCIDevice *d, uint32_t addr,
 
             valid_mask <<= (find_addr - real_offset) << 3;
             ptr_val = (uint8_t *)&val + (real_offset & 3);
+            if (reg->emu_mask == (0xFFFFFFFF >> ((4 - reg->size) << 3))) {
+                wb_mask &= ~((reg->emu_mask
+                              >> ((find_addr - real_offset) << 3))
+                             << ((len - emul_len) << 3));
+            }
 
             /* do emulation based on register size */
             switch (reg->size) {
@@ -350,10 +358,19 @@ static void xen_pt_pci_write_config(PCIDevice *d, uint32_t addr,
     memory_region_transaction_commit();
 
 out:
-    if (!(reg && reg->no_wb)) {
+    for (index = 0; wb_mask; index += len) {
         /* unknown regs are passed through */
-        rc = xen_host_pci_set_block(&s->real_device, addr,
-                                    (uint8_t *)&val, len);
+        while (!(wb_mask & 0xff)) {
+            index++;
+            wb_mask >>= 8;
+        }
+        len = 0;
+        do {
+            len++;
+            wb_mask >>= 8;
+        } while (wb_mask & 0xff);
+        rc = xen_host_pci_set_block(&s->real_device, addr + index,
+                                    (uint8_t *)&val + index, len);
 
         if (rc < 0) {
             XEN_PT_ERR(d, "pci_write_block failed. return value: %d.\n", rc);