Alex, Peter,
On 1/10/19 00:10, Alex Williamson wrote:
A kernel bug was introduced in v4.15 via commit 71a7d3d78e3c which
adds a test for address space wrap-around in the vfio DMA unmap path.
Unfortunately due to overflow, the kernel detects an unmap of the last
page in the 64-bit address space as a wrap-around. In QEMU, a Q35
guest with VT-d emulation and guest IOMMU enabled will attempt to make
such an unmap request during VM system reset, triggering an error:
qemu-kvm: VFIO_UNMAP_DMA: -22
qemu-kvm: vfio_dma_unmap(0x561f059948f0, 0xfef00000, 0xffffffff01100000) =
-22 (Invalid argument)
Here the IOVA start address (0xfef00000) and the size parameter
(0xffffffff01100000) add to exactly 2^64, triggering the bug. A
kernel fix is queued for the Linux v5.0 release to address this.
This patch implements a workaround to retry the unmap, excluding the
final page of the range when we detect an unmap failing which matches
the requirements for this issue. This is expected to be a safe and
complete workaround as the VT-d address space does not extend to the
full 64-bit space and therefore the last page should never be mapped.
This workaround can be removed once all kernels with this bug are
sufficiently deprecated.
Have we waited long enough ? what does "sufficiently deprecated" mean ?
Is it related to the linux stable updates ?
Thanks,
C.
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1662291
Reported-by: Pei Zhang <pezh...@redhat.com>
Debugged-by: Peter Xu <pet...@redhat.com>
Signed-off-by: Alex Williamson <alex.william...@redhat.com>
---
hw/vfio/common.c | 20 +++++++++++++++++++-
hw/vfio/trace-events | 1 +
2 files changed, 20 insertions(+), 1 deletion(-)
diff --git a/hw/vfio/common.c b/hw/vfio/common.c
index 7c185e5a2e79..820b839057c6 100644
--- a/hw/vfio/common.c
+++ b/hw/vfio/common.c
@@ -220,7 +220,25 @@ static int vfio_dma_unmap(VFIOContainer *container,
.size = size,
};
- if (ioctl(container->fd, VFIO_IOMMU_UNMAP_DMA, &unmap)) {
+ while (ioctl(container->fd, VFIO_IOMMU_UNMAP_DMA, &unmap)) {
+ /*
+ * The type1 backend has an off-by-one bug in the kernel (71a7d3d78e3c
+ * v4.15) where an overflow in its wrap-around check prevents us from
+ * unmapping the last page of the address space. Test for the error
+ * condition and re-try the unmap excluding the last page. The
+ * expectation is that we've never mapped the last page anyway and this
+ * unmap request comes via vIOMMU support which also makes it unlikely
+ * that this page is used. This bug was introduced well after type1 v2
+ * support was introduced, so we shouldn't need to test for v1. A fix
+ * is queued for kernel v5.0 so this workaround can be removed once
+ * affected kernels are sufficiently deprecated.
+ */
+ if (errno == EINVAL && unmap.size && !(unmap.iova + unmap.size) &&
+ container->iommu_type == VFIO_TYPE1v2_IOMMU) {
+ trace_vfio_dma_unmap_overflow_workaround();
+ unmap.size -= 1ULL << ctz64(container->pgsizes);
+ continue;
+ }
error_report("VFIO_UNMAP_DMA: %d", -errno);
return -errno;
}
diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
index a85e8662eadb..a002c6af2dda 100644
--- a/hw/vfio/trace-events
+++ b/hw/vfio/trace-events
@@ -110,6 +110,7 @@ vfio_region_mmaps_set_enabled(const char *name, bool enabled)
"Region %s mmaps e
vfio_region_sparse_mmap_header(const char *name, int index, int nr_areas) "Device
%s region %d: %d sparse mmap entries"
vfio_region_sparse_mmap_entry(int i, unsigned long start, unsigned long end)
"sparse entry %d [0x%lx - 0x%lx]"
vfio_get_dev_region(const char *name, int index, uint32_t type, uint32_t subtype)
"%s index %d, %08x/%0x8"
+vfio_dma_unmap_overflow_workaround(void) ""
# hw/vfio/platform.c
vfio_platform_base_device_init(char *name, int groupid) "%s belongs to group
#%d"
