[PATCH V2] KVM: arm64: vgic: fix wrong loop condition in scan_its_table()

[Eric Ren]

Reproducer hints:
1. Create ARM virt VM with pxb-pcie bus which adds
   extra host bridges, with qemu command like:

```
  -device pxb-pcie,bus_nr=8,id=pci.x,numa_node=0,bus=pcie.0 \
  -device pcie-root-port,..,bus=pci.x \
  ...
  -device pxb-pcie,bus_nr=37,id=pci.y,numa_node=1,bus=pcie.0 \
  -device pcie-root-port,..,bus=pci.y \
  ...

```
2. Perform VM migration which calls save/restore device tables;
3. vgic its use 2 level device table.

In that setup, we get a big "offset" between 2 device_ids (
one is small, another is big), which makes unsigned "len" round
up a big positive number, causing loop to continue with a
bad GPA. For example,

1. L1 table has 2 entries;
2. and we are now scanning at L2 table entry index 2075 (pointed by L1
first entry)
3. if next device id is 9472, we will get a big offset: 7397;
4. with signed 'len', 'len -= offset * esz', len will underflow to a
positive number, mistakenly into next iteration with a bad GPA;
(It should break the current L2 table scanning, and jump into the next
L1 table entry)
5. that bad GPA fails the guest read.

Fix it by stopping the L2 table scan when the next device id fits
into next L1 table entry.

Thanks Eric Auger's suggestion for the fix.

Fixes: 920a7a8fa92a ("KVM: arm64: vgic-its: Add infrastructure for tableookup")
Signed-off-by: Eric Ren <renzhengeek@xxxxxxxxx>
---
 arch/arm64/kvm/vgic/vgic-its.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 24d7778..733b530 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -2149,7 +2149,7 @@ static int scan_its_table(struct vgic_its *its, gpa_t base, int size, u32 esz,
 
 	memset(entry, 0, esz);
 
-	while (len > 0) {
+	while (true) {
 		int next_offset;
 		size_t byte_offset;
 
@@ -2162,6 +2162,9 @@ static int scan_its_table(struct vgic_its *its, gpa_t base, int size, u32 esz,
 			return next_offset;
 
 		byte_offset = next_offset * esz;
+		if (byte_offset >= len)
+			break;
+
 		id += next_offset;
 		gpa += byte_offset;
 		len -= byte_offset;
-- 
1.8.3.1