powerpc/prom: Fix "ibm,arch-vec-5-platform-support" scan

The "ibm,arch-vec-5-platform-support" property is a list of pairs of
bytes representing the options and values supported by the platform
firmware. At boot time, Linux scans this list and activates the
available features it recognizes : Radix and XIVE.

A recent change modified the number of entries to loop on and 8 bytes,
4 pairs of { options, values } entries are always scanned. This is
fine on KVM but not on PowerVM which can advertises less. As a
consequence on this platform, Linux reads extra entries pointing to
random data, interprets these as available features and tries to
activate them, leading to a firmware crash in
ibm,client-architecture-support.

Fix that by using the property length of "ibm,arch-vec-5-platform-support".

Fixes: ab912399 ("powerpc/prom: Remove VLA in prom_check_platform_support()")
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210122075029.797013-1-clg@kaod.org

arch/powerpc/kernel/prom_init.c

@@ -1331,14 +1331,10 @@ static void __init prom_check_platform_support(void)
 		if (prop_len > sizeof(vec))
 			prom_printf("WARNING: ibm,arch-vec-5-platform-support longer than expected (len: %d)\n",
 				    prop_len);
-		prom_getprop(prom.chosen, "ibm,arch-vec-5-platform-support",
-			     &vec, sizeof(vec));
-		for (i = 0; i < sizeof(vec); i += 2) {
-			prom_debug("%d: index = 0x%x val = 0x%x\n", i / 2
-								  , vec[i]
-								  , vec[i + 1]);
-			prom_parse_platform_support(vec[i], vec[i + 1],
-						    &supported);
+		prom_getprop(prom.chosen, "ibm,arch-vec-5-platform-support", &vec, sizeof(vec));
+		for (i = 0; i < prop_len; i += 2) {
+			prom_debug("%d: index = 0x%x val = 0x%x\n", i / 2, vec[i], vec[i + 1]);
+			prom_parse_platform_support(vec[i], vec[i + 1], &supported);
 		}
 	}