powerpc/rtas: Prevent Spectre v1 gadget construction in sys_rtas()

commit 0974d03eb479384466d828d65637814bee6b26d7 upstream.

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@linux.ibm.com>
Reported-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240530-sys_rtas-nargs-nret-v1-1-129acddd4d89@linux.ibm.com
[Minor context change fixed]
Signed-off-by: Cliff Liu <donghua.liu@windriver.com>
Signed-off-by: He Zhe <Zhe.He@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 46b9476d75824937fae1352548587396096712a7..dc294c95da21cbf453d0914621feb2c736579dbd 100644 (file)
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -18,6 +18,7 @@
 #include <linux/kernel.h>
 #include <linux/lockdep.h>
 #include <linux/memblock.h>
+#include <linux/nospec.h>
 #include <linux/of.h>
 #include <linux/of_fdt.h>
 #include <linux/reboot.h>
@@ -1839,6 +1840,9 @@ 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)