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

[P J P]

[+Paolo, +Fam Zheng - for scsi]

+-- On Mon, 28 Sep 2020, P J P wrote --+
| +-- 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...?

@Paolo, @Fam...wdyt?

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D