Re: [PATCH] pci: check bus pointer before dereference

[P J P <ppandit@redhat.com>]

  Hello,

+-- On Wed, 16 Sep 2020, Peter Maydell wrote --+
| On Wed, 16 Sep 2020 at 07:28, P J P <ppandit@redhat.com> wrote:
| > -> https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Flsi_nullptr1
| > ==1183858==Hint: address points to the zero page.
| > #0 pci_change_irq_level hw/pci/pci.c:259
| > #1 pci_irq_handler hw/pci/pci.c:1445
| > #2 pci_set_irq hw/pci/pci.c:1463
| > #3 lsi_set_irq hw/scsi/lsi53c895a.c:488
| > #4 lsi_update_irq hw/scsi/lsi53c895a.c:523
| > #5 lsi_script_scsi_interrupt hw/scsi/lsi53c895a.c:554
| > #6 lsi_execute_script hw/scsi/lsi53c895a.c:1149
| > #7 lsi_reg_writeb hw/scsi/lsi53c895a.c:1984
| > #8 lsi_io_write hw/scsi/lsi53c895a.c:2146
...
| Generally we don't bother to assert() that pointers that shouldn't be NULL 
| really are NULL immediately before dereferencing them, because the 
| dereference provides an equally easy-to-debug crash to the assert, and so 
| the assert doesn't provide anything extra. assert()ing that a pointer is 
| non-NULL is more useful if it is done in a place that identifies the problem 
| at an earlier and easier-to-debug point in execution rather than at a later 
| point which is distantly removed from the place where the bogus pointer was 
| introduced.

* The NULL dereference above occurs because the 'pci_dev->qdev->parent_bus' 
  address gets overwritten (with 0x0) during scsi 'Memory Move' operation in

 ../hw/scsi/lsi53c895a.c
  #define LSI_BUF_SIZE 4096

lsi_mmio_write
 lsi_reg_writeb
  lsi_execute_script
   static void lsi_memcpy(LSIState *s, ... int count=12MB)
   {
      int n;
      uint8_t buf[LSI_BUF_SIZE];

      while (count) {
        n = (count > LSI_BUF_SIZE) ? LSI_BUF_SIZE : count;
        lsi_mem_read(s, src, buf, n);          <== read from DMA memory
        lsi_mem_write(s, dest, buf, n);        <== write to I/O memory
        src += n;
        dest += n;
        count -= n;
     }
   }
-> https://www.manualslib.com/manual/1407578/Lsi-Lsi53c895a.html?page=254#manual

* Above loop moves data between DMA memory to i/o address space.

* Going through the manual above, it seems 'Memory Move' can move upto 16MB of 
  data between memory spaces.

* I tried to see a suitable fix, but couldn't get one.

  - Limiting 'count' value does not seem right, as allowed value is upto 16MB.

  - Manual above talks about moving data via 'dma_buf'. But it doesn't seem to 
    be used here.

* During above loop, 'dest' address moves past its 'MemoryRegion mr' and 
  overwrites the adjacent 'mr' memory area, overwritting 'parent_bus' value.

Any thoughts/hints please...?

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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