[Qemu-devel] [PATCH v2] scsi: lsi: exit infinite loop while executing script (CVE-2019-12068)

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

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

When executing script in lsi_execute_script(), the LSI scsi
adapter emulator advances 's->dsp' index to read next opcode.
This can lead to an infinite loop if the next opcode is empty.
Exit such loop after reading 10k empty opcodes.

Reported-by: Bugs SysSec <bugs-syssec@rub.de>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 hw/scsi/lsi53c895a.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Update v2: define LSI_MAX_INSN 10000
  -> https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg01370.html

diff --git a/hw/scsi/lsi53c895a.c b/hw/scsi/lsi53c895a.c
index 10468c1ec1..2adab341b1 100644
--- a/hw/scsi/lsi53c895a.c
+++ b/hw/scsi/lsi53c895a.c
@@ -185,6 +185,9 @@ static const char *names[] = {
 /* Flag set if this is a tagged command.  */
 #define LSI_TAG_VALID     (1 << 16)
 
+/* Maximum instructions to process. */
+#define LSI_MAX_INSN    10000
+
 typedef struct lsi_request {
     SCSIRequest *req;
     uint32_t tag;
@@ -1132,7 +1135,10 @@ static void lsi_execute_script(LSIState *s)
 
     s->istat1 |= LSI_ISTAT1_SRUN;
 again:
-    insn_processed++;
+    if (++insn_processed > LSI_MAX_INSN) {
+        s->waiting = LSI_NOWAIT;
+        goto exitloop;
+    }
     insn = read_dword(s, s->dsp);
     if (!insn) {
         /* If we receive an empty opcode increment the DSP by 4 bytes
@@ -1569,7 +1575,8 @@ again:
             }
         }
     }
-    if (insn_processed > 10000 && s->waiting == LSI_NOWAIT) {
+exitloop:
+    if (insn_processed > LSI_MAX_INSN && s->waiting == LSI_NOWAIT) {
         /* Some windows drivers make the device spin waiting for a memory
            location to change.  If we have been executed a lot of code then
            assume this is the case and force an unexpected device disconnect.
-- 
2.21.0