powerpc/rtas: Allow ibm,platform-dump RTAS call with null buffer address

Add a special case to block_rtas_call() to allow the ibm,platform-dump RTAS
call through the RTAS filter if the buffer address is 0.

According to PAPR, ibm,platform-dump is called with a null buffer address
to notify the platform firmware that processing of a particular dump is
finished.

Without this, on a pseries machine with CONFIG_PPC_RTAS_FILTER enabled, an
application such as rtas_errd that is attempting to retrieve a dump will
encounter an error at the end of the retrieval process.

Fixes: bd59380c ("powerpc/rtas: Restrict RTAS requests from userspace")
Cc: stable@vger.kernel.org
Reported-by: Sathvika Vasireddy <sathvika@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220614134952.156010-1-ajd@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -1071,7 +1071,7 @@ static struct rtas_filter rtas_filters[] __ro_after_init = {
 	{ "get-time-of-day", -1, -1, -1, -1, -1 },
 	{ "ibm,get-vpd", -1, 0, -1, 1, 2 },
 	{ "ibm,lpar-perftools", -1, 2, 3, -1, -1 },
-	{ "ibm,platform-dump", -1, 4, 5, -1, -1 },
+	{ "ibm,platform-dump", -1, 4, 5, -1, -1 },		/* Special cased */
 	{ "ibm,read-slot-reset-state", -1, -1, -1, -1, -1 },
 	{ "ibm,scan-log-dump", -1, 0, 1, -1, -1 },
 	{ "ibm,set-dynamic-indicator", -1, 2, -1, -1, -1 },
@@ -1120,6 +1120,15 @@ static bool block_rtas_call(int token, int nargs,
 				size = 1;
 
 			end = base + size - 1;
+
+			/*
+			 * Special case for ibm,platform-dump - NULL buffer
+			 * address is used to indicate end of dump processing
+			 */
+			if (!strcmp(f->name, "ibm,platform-dump") &&
+			    base == 0)
+				return false;
+
 			if (!in_rmo_buf(base, end))
 				goto err;
 		}