Re: [Bug 1886362] [NEW] Heap use-after-free in lduw_he_p through e1000e_write_to_rx_buffers

[Paolo Bonzini]

On 09/07/20 17:51, Li Qiang wrote:
> Maybe we should check whether the address is a RAM address in 'dma_memory_rw'?
> But it is a hot path. I'm not sure it is right. Hope more discussion.

Half of the purpose of dma-helpers.c (as opposed to address_space_*
functions in exec.c) is exactly to support writes to MMIO.  This is
especially true of dma_blk_io, which takes care of doing the DMA via a
bounce buffer, possibly in multiple steps and even blocking due to
cpu_register_map_client.

For dma_memory_rw this is not needed, so it only needs to handle
QEMUSGList, but I think the design should be the same.

However, this is indeed a nightmare for re-entrancy.  The easiest
solution is to delay processing of descriptors to a bottom half whenever
MMIO is doing something complicated.  This is also better for latency
because it will free the vCPU thread more quickly and leave the work to
the I/O thread.

Paolo