ipmi_ssif: Fix kernel panic at msg_done_handler

BugLink: https://bugs.launchpad.net/bugs/1775771

[ Upstream commit f002612b ]

This happens when BMC doesn't return any data and the code is trying
to print the value of data[2].

Getting following crash:
[  484.728410] Unable to handle kernel NULL pointer dereference at virtual address 00000002
[  484.736496] pgd = ffff0000094a2000
[  484.739885] [00000002] *pgd=00000047fcffe003, *pud=00000047fcffd003, *pmd=0000000000000000
[  484.748158] Internal error: Oops: 96000005 [#1] SMP
[...]
[  485.101451] Call trace:
[...]
[  485.188473] [<ffff000000a46e68>] msg_done_handler+0x668/0x700 [ipmi_ssif]
[  485.195249] [<ffff000000a456b8>] ipmi_ssif_thread+0x110/0x128 [ipmi_ssif]
[  485.202038] [<ffff0000080f1430>] kthread+0x108/0x138
[  485.206994] [<ffff0000080838e0>] ret_from_fork+0x10/0x30
[  485.212294] Code: aa1903e1 aa1803e0 b900227f 95fef6a5 (39400aa3)

Adding a check to validate the data len before printing data[2] to fix this issue.
Signed-off-by: Kamlakant Patel <kamlakant.patel@cavium.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/char/ipmi/ipmi_ssif.c

@@ -757,7 +757,7 @@ static void msg_done_handler(struct ssif_info *ssif_info, int result,
 			ssif_info->ssif_state = SSIF_NORMAL;
 			ipmi_ssif_unlock_cond(ssif_info, flags);
 			pr_warn(PFX "Error getting flags: %d %d, %x\n",
-			       result, len, data[2]);
+			       result, len, (len >= 3) ? data[2] : 0);
 		} else if (data[0] != (IPMI_NETFN_APP_REQUEST | 1) << 2
 			   || data[1] != IPMI_GET_MSG_FLAGS_CMD) {
 			/*
@@ -779,7 +779,7 @@ static void msg_done_handler(struct ssif_info *ssif_info, int result,
 		if ((result < 0) || (len < 3) || (data[2] != 0)) {
 			/* Error clearing flags */
 			pr_warn(PFX "Error clearing flags: %d %d, %x\n",
-			       result, len, data[2]);
+			       result, len, (len >= 3) ? data[2] : 0);
 		} else if (data[0] != (IPMI_NETFN_APP_REQUEST | 1) << 2
 			   || data[1] != IPMI_CLEAR_MSG_FLAGS_CMD) {
 			pr_warn(PFX "Invalid response clearing flags: %x %x\n",