[RFC PATCH-for-5.1? v3 2/2] hw/isa/isa-bus: Ensure ISA I/O regions are 8/16/32-bit accessible

[Philippe Mathieu-Daudé]

Since commit 5d971f9e67 we don't accept mismatching sizes
in memory_region_access_valid(). This gives troubles when
a device is on an ISA bus, because the CPU is free to use
8/16-bit accesses on the bus (or up to 32-bit on EISA bus),
regardless what range is valid for the device.

Monkey-patch the ISA device MemoryRegionOps to force it
to accepts 8/16/32-bit accesses. This should be reverted
after the release and fixed in a more elegant manner.

Related bug reports:

- 
https://lore.kernel.org/xen-devel/20200630170913.123646-1-anthony.perard@citrix.com/T/
- https://bugs.debian.org/964793
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=964247
- https://bugs.launchpad.net/bugs/1886318

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/isa/isa-bus.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/isa/isa-bus.c b/hw/isa/isa-bus.c
index 58fde178f9..c8aed2f55f 100644
--- a/hw/isa/isa-bus.c
+++ b/hw/isa/isa-bus.c
@@ -132,6 +132,20 @@ static inline void isa_init_ioport(ISADevice *dev, 
uint16_t ioport)
 
 void isa_register_ioport(ISADevice *dev, MemoryRegion *io, uint16_t start)
 {
+    if (io->ops->valid.min_access_size > 1 ||
+        io->ops->valid.max_access_size < 4) {
+        warn_report_once("Monkey-patching ISA I/O access sizes "
+                         "(side effect of CVE-2020-13754, only for QEMU 
v5.1)");
+        /*
+         * To be backward compatible with IBM-PC bus, ISA bus must accept
+         * 8-bit accesses.
+         */
+        io->ops->valid.min_access_size = 1;
+        /*
+         * EISA bus must accept 32-bit accesses.
+         */
+        io->ops->valid.max_access_size = 4;
+    }
     memory_region_add_subregion(isabus->address_space_io, start, io);
     isa_init_ioport(dev, start);
 }
-- 
2.21.3