In the Linux kernel, the following vulnerability has been resolved:
uiohvgeneric: Let userspace take care of interrupt mask
Remove the logic to set interrupt mask by default in uiohvgeneric driver as the interrupt mask value is supposed to be controlled completely by the user space. If the mask bit gets changed by the driver, concurrently with user mode operating on the ring, the mask bit may be set when it is supposed to be clear, and the user-mode driver will miss an interrupt which will cause a hang.
For eg- when the driver sets inbound ring buffer interrupt mask to 1, the host does not interrupt the guest on the UIO VMBus channel. However, setting the mask does not prevent the host from putting a message in the inbound ring buffer. So let’s assume that happens, the host puts a message into the ring buffer but does not interrupt.
Subsequently, the user space code in the guest sets the inbound ring buffer interrupt mask to 0, saying “Hey, I’m ready for interrupts”. User space code then calls pread() to wait for an interrupt. Then one of two things happens:
This could be easily reproduced in hvfcopyuio_daemon if we delay setting interrupt mask to 0.
Similarly if hvuiochannelcb() sets the interruptmask to 1, there’s a race condition. Once user space empties the inbound ring buffer, but before user space sets interrupt_mask to 0, the host could put another message in the ring buffer but it wouldn’t interrupt. Then the next pread() would hang.
Fix these by removing all instances where interruptmask is changed, while keeping the one in setevent() unchanged to enable userspace control the interrupt mask by writing 0/1 to /dev/uioX.