core/ipmi: Fix use-after-free

Commit f01cd77 introduced backend poller() for ipmi message. But in some
corner cases its possible that we endup calling poller() after freeing
ipmi message.

Thread 1 :
  ipmi_queue_msg_sync()
    Waiting for ipmi sync message to complete

Thread 2 :
  bt_poll() -> ipmi_cmd_done() -> callback handler -> free message

Oliver hit this issue during fast-reboot test with skiboot DEBUG build.
In debug build we poision the memory after free. That helped us to catch
this issue.

[  460.295570781,3] ***********************************************
[  460.295773157,3] Fatal MCE at 0000000030035cb4   .ipmi_queue_msg_sync+0x110  MSR 9000000000201002
[  460.295887496,3] CFAR : 0000000030035ce8 MSR  : 9000000000000000
[  460.295956419,3] SRR0 : 0000000030035cb4 SRR1 : 9000000000201002
[  460.296035015,3] HSRR0: 0000000030012624 HSRR1: 9000000002803002
[  460.296102413,3] DSISR: 00000008         DAR  : 99999999999999d1
[  460.296169710,3] LR   : 0000000030035ce4 CTR  : 0000000030002880
[  460.296248482,3] CR   : 28002422         XER  : 20040000
[  460.296336621,3] GPR00: 0000000030035ce4 GPR16: 00000000301d36d8
[  460.296415449,3] GPR01: 0000000031c133d0 GPR17: 00000000300f5cd8
[  460.296482811,3] GPR02: 0000000030142700 GPR18: 0000000030407ff0
[  460.296550265,3] GPR03: 0000000000000100 GPR19: 0000000000000000
[  460.296629041,3] GPR04: 0000000028002424 GPR20: 0000000000000000
[  460.296696369,3] GPR05: 0000000020040000 GPR21: 0000000030121d73
[  460.296820977,3] GPR06: c000001fffffd480 GPR22: 0000000030121dd2
[  460.296888226,3] GPR07: c000001fffffd480 GPR23: 0000000030613400
[  460.296978218,3] GPR08: 0000000000000001 GPR24: 0000000000000001
[  460.297056871,3] GPR09: 9999999999999999 GPR25: 0000000031c13960
[  460.297124647,3] GPR10: 0000000000000000 GPR26: 0000000000000004
[  460.297203811,3] GPR11: 0000000000000000 GPR27: 0000000000000003
[  460.297271250,3] GPR12: 0000000028002424 GPR28: 0000000030613400
[  460.297339026,3] GPR13: 0000000031c10000 GPR29: 0000000030406b50
[  460.297417605,3] GPR14: 00000000300f58f8 GPR30: 0000000030406b40
[  460.297485176,3] GPR15: 00000000300f58d8 GPR31: 00000000309249c8

Reported-by: Oliver O'Halloran <oohall@gmail.com>
Fixes: f01cd77 (ipmi: ensure forward progress on ipmi_queue_msg_sync())
Cc: skiboot-stable@lists.ozlabs.org # v6.3+
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

core/ipmi.c

@@ -126,6 +126,8 @@ int ipmi_dequeue_msg(struct ipmi_msg *msg)
 
 void ipmi_cmd_done(uint8_t cmd, uint8_t netfn, uint8_t cc, struct ipmi_msg *msg)
 {
+	bool clr_sync_msg = false;
+
 	msg->cc = cc;
 	if (msg->cmd != cmd) {
 		prerror("IPMI: Incorrect cmd 0x%02x in response\n", cmd);
@@ -138,6 +140,10 @@ void ipmi_cmd_done(uint8_t cmd, uint8_t netfn, uint8_t cc, struct ipmi_msg *msg)
 	}
 	msg->netfn = netfn;
 
+	if (msg == sync_msg)
+		clr_sync_msg = true;
+
+
 	if (cc != IPMI_CC_NO_ERROR) {
 		prlog(PR_DEBUG, "IPMI: Got error response. cmd=0x%x, netfn=0x%x,"
 		      " rc=0x%02x\n", msg->cmd, msg->netfn >> 2, msg->cc);
@@ -151,12 +157,14 @@ void ipmi_cmd_done(uint8_t cmd, uint8_t netfn, uint8_t cc, struct ipmi_msg *msg)
 	   completion functions. */
 
 	/* If this is a synchronous message flag that we are done */
-	if (msg == sync_msg)
+	if (clr_sync_msg)
 		sync_msg = NULL;
 }
 
 void ipmi_queue_msg_sync(struct ipmi_msg *msg)
 {
+	void (*poll)(void) = msg->backend->poll;
+
 	if (!ipmi_present())
 		return;
 
@@ -181,8 +189,8 @@ void ipmi_queue_msg_sync(struct ipmi_msg *msg)
 	 * progress.
 	 */
 	while (sync_msg == msg) {
-		if (msg->backend->poll)
-			msg->backend->poll();
+		if (poll)
+			poll();
 		time_wait_ms(10);
 	}
 }