[Qemu-devel] [PATCH v3] scsi: pvscsi: avoid infinite loop while building SG list

[P J P <ppandit@redhat.com>]

From: Prasad J Pandit <pjp@fedoraproject.org>

In PVSCSI paravirtual SCSI bus, pvscsi_convert_sglist can take a very
long time or go into an infinite loop due to two different bugs:

1) the request descriptor data length is defined to be 64 bit. While
building SG list from a request descriptor, it gets truncated to 32bit
in routine 'pvscsi_convert_sglist'. This could lead to an infinite loop
situation for large 'dataLen' values, when data_length is cast to uint32_t
and chunk_size becomes always zero.  Fix this by removing the incorrect
cast.

2) pvscsi_get_next_sg_elem can be called arbitrarily many times if the
element has a zero length.  Get out of the loop early when this happens,
by introducing an upper limit on the number of SG list elements.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 hw/scsi/vmw_pvscsi.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

Update as per:
  -> https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg01172.html

diff --git a/hw/scsi/vmw_pvscsi.c b/hw/scsi/vmw_pvscsi.c
index 4245c15..babac5a 100644
--- a/hw/scsi/vmw_pvscsi.c
+++ b/hw/scsi/vmw_pvscsi.c
@@ -40,6 +40,8 @@
 #define PVSCSI_MAX_DEVS                   (64)
 #define PVSCSI_MSIX_NUM_VECTORS           (1)
 
+#define PVSCSI_MAX_SG_ELEM                2048
+
 #define PVSCSI_MAX_CMD_DATA_WORDS \
     (sizeof(PVSCSICmdDescSetupRings)/sizeof(uint32_t))
 
@@ -628,17 +630,16 @@ pvscsi_queue_pending_descriptor(PVSCSIState *s, SCSIDevice **d,
 static void
 pvscsi_convert_sglist(PVSCSIRequest *r)
 {
-    int chunk_size;
+    uint32_t chunk_size, elmcnt = 0;
     uint64_t data_length = r->req.dataLen;
     PVSCSISGState sg = r->sg;
-    while (data_length) {
-        while (!sg.resid) {
+    while (data_length && elmcnt < PVSCSI_MAX_SG_ELEM) {
+        while (!sg.resid && elmcnt++ < PVSCSI_MAX_SG_ELEM) {
             pvscsi_get_next_sg_elem(&sg);
             trace_pvscsi_convert_sglist(r->req.context, r->sg.dataAddr,
                                         r->sg.resid);
         }
-        assert(data_length > 0);
-        chunk_size = MIN((unsigned) data_length, sg.resid);
+        chunk_size = MIN(data_length, sg.resid);
         if (chunk_size) {
             qemu_sglist_add(&r->sgl, sg.dataAddr, chunk_size);
         }
-- 
2.5.5