s390: fix stfle zero padding

The stfle inline assembly returns the number of double words written
(condition code 0) or the double words it would have written
(condition code 3), if the memory array it got as parameter would have
been large enough.

The current stfle implementation assumes that the array is always
large enough and clears those parts of the array that have not been
written to with a subsequent memset call.

If however the array is not large enough memset will get a negative
length parameter, which means that memset clears memory until it gets
an exception and the kernel crashes.

To fix this simply limit the maximum length. Move also the inline
assembly to an extra function to avoid clobbering of register 0, which
might happen because of the added min_t invocation together with code
instrumentation.

The bug was introduced with commit 14375bc4 ("[S390] cleanup
facility list handling") but was rather harmless, since it would only
write to a rather large array. It became a potential problem with
commit 3ab121ab ("[S390] kernel: Add z/VM LGR detection"). Since
then it writes to an array with only four double words, while some
machines already deliver three double words. As soon as machines have
a facility bit within the fifth double a crash on IPL would happen.

Fixes: 14375bc4 ("[S390] cleanup facility list handling")
Cc: <stable@vger.kernel.org> # v2.6.37+
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/facility.h

@@ -59,6 +59,18 @@ static inline int test_facility(unsigned long nr)
 	return __test_facility(nr, &S390_lowcore.stfle_fac_list);
 }
 
+static inline unsigned long __stfle_asm(u64 *stfle_fac_list, int size)
+{
+	register unsigned long reg0 asm("0") = size - 1;
+
+	asm volatile(
+		".insn s,0xb2b00000,0(%1)" /* stfle */
+		: "+d" (reg0)
+		: "a" (stfle_fac_list)
+		: "memory", "cc");
+	return reg0;
+}
+
 /**
  * stfle - Store facility list extended
  * @stfle_fac_list: array where facility list can be stored
@@ -75,13 +87,8 @@ static inline void __stfle(u64 *stfle_fac_list, int size)
 	memcpy(stfle_fac_list, &S390_lowcore.stfl_fac_list, 4);
 	if (S390_lowcore.stfl_fac_list & 0x01000000) {
 		/* More facility bits available with stfle */
-		register unsigned long reg0 asm("0") = size - 1;
-
-		asm volatile(".insn s,0xb2b00000,0(%1)" /* stfle */
-			     : "+d" (reg0)
-			     : "a" (stfle_fac_list)
-			     : "memory", "cc");
-		nr = (reg0 + 1) * 8; /* # bytes stored by stfle */
+		nr = __stfle_asm(stfle_fac_list, size);
+		nr = min_t(unsigned long, (nr + 1) * 8, size * 8);
 	}
 	memset((char *) stfle_fac_list + nr, 0, size * 8 - nr);
 }