ipmi: msghandler: Fix potential Spectre v1 vulnerabilities

channel and addr->channel are indirectly controlled by user-space,
hence leading to a potential exploitation of the Spectre variant 1
vulnerability.

These issues were detected with the help of Smatch:

drivers/char/ipmi/ipmi_msghandler.c:1381 ipmi_set_my_address() warn: potential spectre issue 'user->intf->addrinfo' [w] (local cap)
drivers/char/ipmi/ipmi_msghandler.c:1401 ipmi_get_my_address() warn: potential spectre issue 'user->intf->addrinfo' [r] (local cap)
drivers/char/ipmi/ipmi_msghandler.c:1421 ipmi_set_my_LUN() warn: potential spectre issue 'user->intf->addrinfo' [w] (local cap)
drivers/char/ipmi/ipmi_msghandler.c:1441 ipmi_get_my_LUN() warn: potential spectre issue 'user->intf->addrinfo' [r] (local cap)
drivers/char/ipmi/ipmi_msghandler.c:2260 check_addr() warn: potential spectre issue 'intf->addrinfo' [r] (local cap)

Fix this by sanitizing channel and addr->channel before using them to
index user->intf->addrinfo and intf->addrinfo, correspondingly.

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://lore.kernel.org/lkml/20180423164740.GY17484@dhcp22.suse.cz/

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

CVE-2017-5753

(backported from commit a7102c74)
[juergh: Adjused context.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -46,6 +46,7 @@
 #include <linux/proc_fs.h>
 #include <linux/rcupdate.h>
 #include <linux/interrupt.h>
+#include <linux/nospec.h>
 
 #define PFX "IPMI message handler: "
 
@@ -1132,6 +1133,7 @@ int ipmi_set_my_address(ipmi_user_t   user,
 {
 	if (channel >= IPMI_MAX_CHANNELS)
 		return -EINVAL;
+	channel = array_index_nospec(channel, IPMI_MAX_CHANNELS);
 	user->intf->channels[channel].address = address;
 	return 0;
 }
@@ -1143,6 +1145,7 @@ int ipmi_get_my_address(ipmi_user_t   user,
 {
 	if (channel >= IPMI_MAX_CHANNELS)
 		return -EINVAL;
+	channel = array_index_nospec(channel, IPMI_MAX_CHANNELS);
 	*address = user->intf->channels[channel].address;
 	return 0;
 }
@@ -1154,6 +1157,7 @@ int ipmi_set_my_LUN(ipmi_user_t   user,
 {
 	if (channel >= IPMI_MAX_CHANNELS)
 		return -EINVAL;
+	channel = array_index_nospec(channel, IPMI_MAX_CHANNELS);
 	user->intf->channels[channel].lun = LUN & 0x3;
 	return 0;
 }
@@ -1165,6 +1169,7 @@ int ipmi_get_my_LUN(ipmi_user_t   user,
 {
 	if (channel >= IPMI_MAX_CHANNELS)
 		return -EINVAL;
+	channel = array_index_nospec(channel, IPMI_MAX_CHANNELS);
 	*address = user->intf->channels[channel].lun;
 	return 0;
 }
@@ -1926,6 +1931,7 @@ static int check_addr(ipmi_smi_t       intf,
 {
 	if (addr->channel >= IPMI_MAX_CHANNELS)
 		return -EINVAL;
+	addr->channel = array_index_nospec(addr->channel, IPMI_MAX_CHANNELS);
 	*lun = intf->channels[addr->channel].lun;
 	*saddr = intf->channels[addr->channel].address;
 	return 0;