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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 30 Sep 2020 10:32:42 +0530 (IST)
P J P <ppandit@redhat.com> wrote:

> [+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...?

'dest' is offset into MemoryRegion, so far I don't see how it could break into QEMU stack.
Do you have a simple reproducer?

> @Paolo, @Fam...wdyt?
> 
> Thank you.
> --
> Prasad J Pandit / Red Hat Product Security Team
> 8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D