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

[Alexander Bulekov]

On 220621 1630, Peter Maydell wrote:
> 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.

Both of these handle the BH->DMA->MMIO case. Unlike MMIO, I don't think
there is any neat way to set the engaged_in_io flag as we enter a BH. So
instead, we try to set it when a device initiates DMA.

The pci function lets us do that since we get a glimpse of the dev/qdev
(unlike the dma_memory_...  functions).

> 
> 
> > 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?

This one has the same goal, but accesses the qdev through sg, instead of
PCI.
-Alex


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