Re: [PATCH v7 14/17] vfio-user: handle PCI BAR accesses

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 29, 2022 at 03:51:17PM +0000, Jag Raman wrote:
> 
> 
> > On Mar 29, 2022, at 8:50 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Fri, Mar 25, 2022 at 03:19:43PM -0400, Jagannathan Raman wrote:
> >> @@ -324,6 +325,170 @@ static void dma_unregister(vfu_ctx_t *vfu_ctx, vfu_dma_info_t *info)
> >>     trace_vfu_dma_unregister((uint64_t)info->iova.iov_base);
> >> }
> >> 
> >> +static size_t vfu_object_bar_rw(PCIDevice *pci_dev, int pci_bar,
> >> +                                hwaddr offset, char * const buf,
> >> +                                hwaddr len, const bool is_write)
> >> +{
> >> +    uint8_t *ptr = (uint8_t *)buf;
> >> +    uint8_t *ram_ptr = NULL;
> >> +    bool release_lock = false;
> >> +    MemoryRegionSection section = { 0 };
> >> +    MemoryRegion *mr = NULL;
> >> +    int access_size;
> >> +    hwaddr size = 0;
> >> +    MemTxResult result;
> >> +    uint64_t val;
> >> +
> >> +    section = memory_region_find(pci_dev->io_regions[pci_bar].memory,
> >> +                                 offset, len);
> >> +
> >> +    if (!section.mr) {
> >> +        return 0;
> >> +    }
> >> +
> >> +    mr = section.mr;
> >> +    offset = section.offset_within_region;
> >> +
> >> +    if (is_write && mr->readonly) {
> >> +        warn_report("vfu: attempting to write to readonly region in "
> >> +                    "bar %d - [0x%"PRIx64" - 0x%"PRIx64"]",
> >> +                    pci_bar, offset, (offset + len));
> >> +        return 0;
> > 
> > A mr reference is leaked. The return statement can be replaced with goto
> > exit.
> 
> OK.
> 
> > 
> >> +    }
> >> +
> >> +    if (memory_access_is_direct(mr, is_write)) {
> >> +        /**
> >> +         * Some devices expose a PCI expansion ROM, which could be buffer
> >> +         * based as compared to other regions which are primarily based on
> >> +         * MemoryRegionOps. memory_region_find() would already check
> >> +         * for buffer overflow, we don't need to repeat it here.
> >> +         */
> >> +        ram_ptr = memory_region_get_ram_ptr(mr);
> >> +
> >> +        size = len;
> > 
> > This looks like it will access beyond the end of ram_ptr when
> > section.size < len after memory_region_find() returns.
> 
> OK - will will handle shorter sections returned by memory_region_find().
> 
> > 
> >> +
> >> +        if (is_write) {
> >> +            memcpy((ram_ptr + offset), buf, size);
> >> +        } else {
> >> +            memcpy(buf, (ram_ptr + offset), size);
> >> +        }
> >> +
> >> +        goto exit;
> > 
> > What happens when the access spans two adjacent MemoryRegions? I think
> > the while (len > 0) loop is needed even in the memory_access_is_direct()
> > case so we perform the full access instead of truncating it.
> 
> OK - this should be covered by the shorter section handling above.

No, shorter section handling truncates that access. I think the caller
expects all len bytes to be accessed, not just the first few bytes?

Stefan

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