[PATCH 6.1.y] powerpc/rtas: Prevent Spectre v1 gadget construction in sys_rtas()

[Cliff Liu]

From: Nathan Lynch <nathanl@xxxxxxxxxxxxx>

[ Upstream commit 0974d03eb479384466d828d65637814bee6b26d7 ]

Smatch warns:

  arch/powerpc/kernel/rtas.c:1932 __do_sys_rtas() warn: potential
  spectre issue 'args.args' [r] (local cap)

The 'nargs' and 'nret' locals come directly from a user-supplied
buffer and are used as indexes into a small stack-based array and as
inputs to copy_to_user() after they are subject to bounds checks.

Use array_index_nospec() after the bounds checks to clamp these values
for speculative execution.

Signed-off-by: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
Reviewed-by: Breno Leitao <leitao@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://msgid.link/20240530-sys_rtas-nargs-nret-v1-1-129acddd4d89@xxxxxxxxxxxxx
[Minor context change fixed]
Signed-off-by: Cliff Liu <donghua.liu@xxxxxxxxxxxxx>
Signed-off-by: He Zhe <Zhe.He@xxxxxxxxxxxxx>
---
Verified the powerpc build test.
---
 arch/powerpc/kernel/rtas.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index f8d3caad4cf3..3c06c8389e05 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -25,6 +25,7 @@
 #include <linux/reboot.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
+#include <linux/nospec.h>
 #include <linux/of.h>
 #include <linux/of_fdt.h>
 
@@ -1178,6 +1179,9 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
 	    || nargs + nret > ARRAY_SIZE(args.args))
 		return -EINVAL;
 
+	nargs = array_index_nospec(nargs, ARRAY_SIZE(args.args));
+	nret = array_index_nospec(nret, ARRAY_SIZE(args.args) - nargs);
+
 	/* Copy in args. */
 	if (copy_from_user(args.args, uargs->args,
 			   nargs * sizeof(rtas_arg_t)) != 0)
-- 
2.34.1