[RFC] hw/net/vmxnet3: allow VMXNET3_MAX_MTU itself as a value

[Fiona Ebner <f.ebner@proxmox.com>]

Fixes: d05dcd94ae ("net: vmxnet3: validate configuration values during activate (CVE-2021-20203)")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

I'm not familiar with this code, so really I'm asking: is the change
justified?

I tested the change and it seems to work, but I only have some rough
rationale for it, which is also why there's no commit message yet.

In the Linux kernel's net/core/dev.c, in dev_validate_mtu(), the upper
limit itself is a valid value:
    if (dev->max_mtu > 0 && new_mtu > dev->max_mtu) {
        NL_SET_ERR_MSG(extack, "mtu greater than device maximum");
        return -EINVAL;
    }
and AFAICT in the case of the vmxnet3 driver, max_mtu is set to
VMXNET3_MAX_MTU (as defined in the kernel, which is 9000, same as in
QEMU).

Reported by one of our users running into the failing assert():
https://forum.proxmox.com/threads/114011/#post-492916

 hw/net/vmxnet3.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/net/vmxnet3.c b/hw/net/vmxnet3.c
index 0b7acf7f89..a2037583bf 100644
--- a/hw/net/vmxnet3.c
+++ b/hw/net/vmxnet3.c
@@ -1441,7 +1441,7 @@ static void vmxnet3_activate_device(VMXNET3State *s)
     vmxnet3_setup_rx_filtering(s);
     /* Cache fields from shared memory */
     s->mtu = VMXNET3_READ_DRV_SHARED32(d, s->drv_shmem, devRead.misc.mtu);
-    assert(VMXNET3_MIN_MTU <= s->mtu && s->mtu < VMXNET3_MAX_MTU);
+    assert(VMXNET3_MIN_MTU <= s->mtu && s->mtu <= VMXNET3_MAX_MTU);
     VMW_CFPRN("MTU is %u", s->mtu);
 
     s->max_rx_frags =
-- 
2.30.2