On 220621 1630, Peter Maydell wrote:
> On Thu, 9 Jun 2022 at 14:59, Alexander Bulekov <alx...@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-jva6w38lk4hjowae5kcknhprd5fp...@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 <alx...@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
