firmware: arm_scmi: Fix possible scmi_linux_errmap buffer overflow

stable inclusion
from linux-4.19.200
commit edb6f5a95a0c7cdd40c91db952a63c915375e876

--------------------------------

[ Upstream commit 7a691f16 ]

The scmi_linux_errmap buffer access index is supposed to depend on the
array size to prevent element out of bounds access. It uses SCMI_ERR_MAX
to check bounds but that can mismatch with the array size. It also
changes the success into -EIO though scmi_linux_errmap is never used in
case of success, it is expected to work for success case too.

It is slightly confusing code as the negative of the error code
is used as index to the buffer. Fix it by negating it at the start and
make it more readable.

Link: https://lore.kernel.org/r/20210707135028.1869642-1-sudeep.holla@arm.comReported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/firmware/arm_scmi/driver.c

@@ -48,7 +48,6 @@ enum scmi_error_codes {
 	SCMI_ERR_GENERIC = -8,	/* Generic Error */
 	SCMI_ERR_HARDWARE = -9,	/* Hardware Error */
 	SCMI_ERR_PROTOCOL = -10,/* Protocol Error */
-	SCMI_ERR_MAX
 };
 
 /* List of all SCMI devices active in system */
@@ -168,8 +167,10 @@ static const int scmi_linux_errmap[] = {
 
 static inline int scmi_to_linux_errno(int errno)
 {
-	if (errno < SCMI_SUCCESS && errno > SCMI_ERR_MAX)
-		return scmi_linux_errmap[-errno];
+	int err_idx = -errno;
+
+	if (err_idx >= SCMI_SUCCESS && err_idx < ARRAY_SIZE(scmi_linux_errmap))
+		return scmi_linux_errmap[err_idx];
 	return -EIO;
 }