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

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

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 8064d9c3de8620d27d9c87f829676ef048aeed40..f7e86e09c49fa129b0783a68d1de081509beea54 100644 (file)
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -19,6 +19,7 @@
 #include <linux/lockdep.h>
 #include <linux/memblock.h>
 #include <linux/mutex.h>
+#include <linux/nospec.h>
 #include <linux/of.h>
 #include <linux/of_fdt.h>
 #include <linux/reboot.h>
@@ -1916,6 +1917,9 @@ 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)