[PATCH] powerpc/kernel: Fix potential spectre v1 in syscall

[Breno Leitao <leitao@debian.org>]

The rtas syscall reads a value from a user-provided structure and uses it
to index an array, being a possible area for a potential spectre v1 attack.
This is the code that exposes this problem.

	args.rets = &args.args[nargs];

The nargs is an user provided value, and the below code is an example where
the 'nargs' value would be set to XX.

	struct rtas_args ra;
	ra.nargs = htobe32(XX);
	syscall(__NR_rtas, &ra);

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 arch/powerpc/kernel/rtas.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 8afd146bc9c7..5ef3c863003d 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -27,6 +27,7 @@
 #include <linux/slab.h>
 #include <linux/reboot.h>
 #include <linux/syscalls.h>
+#include <linux/nospec.h>
 
 #include <asm/prom.h>
 #include <asm/rtas.h>
@@ -1056,7 +1057,7 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
 	struct rtas_args args;
 	unsigned long flags;
 	char *buff_copy, *errbuf = NULL;
-	int nargs, nret, token;
+	int index, nargs, nret, token;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -1084,7 +1085,8 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
 	if (token == RTAS_UNKNOWN_SERVICE)
 		return -EINVAL;
 
-	args.rets = &args.args[nargs];
+	index = array_index_nospec(nargs, ARRAY_SIZE(args.args));
+	args.rets = &args.args[index];
 	memset(args.rets, 0, nret * sizeof(rtas_arg_t));
 
 	/* Need to handle ibm,suspend_me call specially */
-- 
2.16.3