s390/sclp: Fix SCCB present check

Tracing code called by the SCLP interrupt handler contains early exits
if the SCCB address associated with an interrupt is NULL. This check is
performed after physical to virtual address translation.

If the kernel identity mapping does not start at address zero, the
resulting virtual address is never zero, so that the NULL checks won't
work. Subsequently this may result in incorrect accesses to the first
page of the identity mapping.

Fix this by introducing a function that handles the NULL case before
address translation.

Fixes: ada1da31ce34 ("s390/sclp: sort out physical vs virtual pointers usage")
Cc: [e-mail]
Reviewed-by: Alexander Gordeev <[e-mail]>
Signed-off-by: Peter Oberparleiter <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>

drivers/s390/char/sclp.c

···
 /* The currently active SCLP command word. */
 static sclp_cmdw_t active_cmd;
 
+static inline struct sccb_header *sclpint_to_sccb(u32 sccb_int)
+{
+	if (sccb_int)
+		return __va(sccb_int);
+	return NULL;
+}
+
 static inline void sclp_trace(int prio, char *id, u32 a, u64 b, bool err)
 {
 	struct sclp_trace_entry e;
···
 
 static bool ok_response(u32 sccb_int, sclp_cmdw_t cmd)
 {
-	struct sccb_header *sccb = (struct sccb_header *)__va(sccb_int);
+	struct sccb_header *sccb = sclpint_to_sccb(sccb_int);
 	struct evbuf_header *evbuf;
 	u16 response;
 
···
 
 	/* INT: Interrupt received (a=intparm, b=cmd) */
 	sclp_trace_sccb(0, "INT", param32, active_cmd, active_cmd,
-			(struct sccb_header *)__va(finished_sccb),
+			sclpint_to_sccb(finished_sccb),
 			!ok_response(finished_sccb, active_cmd));
 
 	if (finished_sccb) {