drivers/misc: sram-exec: have the callers of set_memory_*() check the return value

Right now several architectures allow their set_memory_*() family of
functions to fail, but callers may not be checking the return values.
If set_memory_*() returns with an error, call-site assumptions may be
infact wrong to assume that it would either succeed or not succeed at
all. Ideally, the failure of set_memory_*() should be passed up the call
stack, and callers should examine the failure and deal with it.

Need to fix the callers and add the __must_check attribute. They also
may not provide any level of atomicity, in the sense that the memory
protections may be left incomplete on failure. This issue likely has a
few steps on effects architectures:
  1) Have all callers of set_memory_*() helpers check the return value.
  2) Add __must_check to all set_memory_*() helpers so that new uses do
     not ignore the return value.
  3) Add atomicity to the calls so that the memory protections aren't
     left in a partial state.

This series is part of step 1. Make sram driver check the return value
of set_memory_*().

Signed-off-by: Tianlin Li <tli@digitalocean.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191217194528.16461-1-tli@digitalocean.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/sram-exec.c

@@ -85,6 +85,7 @@ void *sram_exec_copy(struct gen_pool *pool, void *dst, void *src,
 	unsigned long base;
 	int pages;
 	void *dst_cpy;
+	int ret;
 
 	mutex_lock(&exec_pool_list_mutex);
 	list_for_each_entry(p, &exec_pool_list, list) {
@@ -104,16 +105,28 @@ void *sram_exec_copy(struct gen_pool *pool, void *dst, void *src,
 
 	mutex_lock(&part->lock);
 
-	set_memory_nx((unsigned long)base, pages);
-	set_memory_rw((unsigned long)base, pages);
+	ret = set_memory_nx((unsigned long)base, pages);
+	if (ret)
+		goto error_out;
+	ret = set_memory_rw((unsigned long)base, pages);
+	if (ret)
+		goto error_out;
 
 	dst_cpy = fncpy(dst, src, size);
 
-	set_memory_ro((unsigned long)base, pages);
-	set_memory_x((unsigned long)base, pages);
+	ret = set_memory_ro((unsigned long)base, pages);
+	if (ret)
+		goto error_out;
+	ret = set_memory_x((unsigned long)base, pages);
+	if (ret)
+		goto error_out;
 
 	mutex_unlock(&part->lock);
 
 	return dst_cpy;
+
+error_out:
+	mutex_unlock(&part->lock);
+	return NULL;
 }
 EXPORT_SYMBOL_GPL(sram_exec_copy);