Re: [PATCH v2] memory: prevent dma-reentracy issues

[Peter Maydell]

On Thu, 9 Jun 2022 at 14:59, Alexander Bulekov <alxndr@bu.edu> wrote:
>
> Add a flag to the DeviceState, when a device is engaged in PIO/MMIO/DMA.
> This flag is set/checked prior to calling a device's MemoryRegion
> handlers, and set when device code initiates DMA.  The purpose of this
> flag is to prevent two types of DMA-based reentrancy issues:
>
> 1.) mmio -> dma -> mmio case
> 2.) bh -> dma write -> mmio case
>
> These issues have led to problems such as stack-exhaustion and
> use-after-frees.
>
> Summary of the problem from Peter Maydell:
> https://lore.kernel.org/qemu-devel/CAFEAcA_23vc7hE3iaM-JVA6W38LK4hJoWae5KcknhPRD5fPBZA@mail.gmail.com
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/62
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/540
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/541
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/556
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/557
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/827
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  include/hw/pci/pci.h   | 13 +++++++++++--
>  include/hw/qdev-core.h |  3 +++
>  softmmu/dma-helpers.c  | 12 ++++++++++++
>  softmmu/memory.c       | 15 +++++++++++++++
>  softmmu/trace-events   |  1 +
>  5 files changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> index 44dacfa224..ab1ad0f7a8 100644
> --- a/include/hw/pci/pci.h
> +++ b/include/hw/pci/pci.h
> @@ -834,8 +834,17 @@ static inline MemTxResult pci_dma_rw(PCIDevice *dev, 
> dma_addr_t addr,
>                                       void *buf, dma_addr_t len,
>                                       DMADirection dir, MemTxAttrs attrs)
>  {
> -    return dma_memory_rw(pci_get_address_space(dev), addr, buf, len,
> -                         dir, attrs);
> +    bool prior_engaged_state;
> +    MemTxResult result;
> +
> +    prior_engaged_state = dev->qdev.engaged_in_io;
> +
> +    dev->qdev.engaged_in_io = true;
> +    result = dma_memory_rw(pci_get_address_space(dev), addr, buf, len,
> +                           dir, attrs);
> +    dev->qdev.engaged_in_io = prior_engaged_state;
> +
> +    return result;

Why do we need to do something in this pci-specific function ?
I was expecting this to only need changes at the generic-to-all-devices
level.


> diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
> index 7820fec54c..7a4f1fb9b3 100644
> --- a/softmmu/dma-helpers.c
> +++ b/softmmu/dma-helpers.c
> @@ -288,8 +288,16 @@ static MemTxResult dma_buf_rw(void *buf, dma_addr_t len, 
> dma_addr_t *residual,
>      uint8_t *ptr = buf;
>      dma_addr_t xresidual;
>      int sg_cur_index;
> +    DeviceState *dev;
> +    bool prior_engaged_state;
>      MemTxResult res = MEMTX_OK;
>
> +    dev = sg->dev;
> +    if (dev) {
> +        prior_engaged_state = dev->engaged_in_io;
> +        dev->engaged_in_io = true;
> +    }
> +
>      xresidual = sg->size;
>      sg_cur_index = 0;
>      len = MIN(len, xresidual);
> @@ -302,6 +310,10 @@ static MemTxResult dma_buf_rw(void *buf, dma_addr_t len, 
> dma_addr_t *residual,
>          xresidual -= xfer;
>      }
>
> +    if (dev) {
> +        dev->engaged_in_io = prior_engaged_state;
> +    }

Not all DMA goes through dma_buf_rw() -- why does it need changes?

> +
>      if (residual) {
>          *residual = xresidual;
>      }

thanks
-- PMM