lib/stackdepot: kmsan: mark API outputs as initialized

KMSAN does not instrument stackdepot and may treat memory allocated by it
as uninitialized.  This is not a problem for KMSAN itself, because its
functions calling stackdepot API are also not instrumented.  But other
kernel features (e.g.  netdev tracker) may access stack depot from
instrumented code, which will lead to false positives, unless we
explicitly mark stackdepot outputs as initialized.

Link: https://lkml.kernel.org/r/20230306111322.205724-1-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/stackdepot.c

@@ -17,6 +17,7 @@
 #include <linux/gfp.h>
 #include <linux/jhash.h>
 #include <linux/kernel.h>
+#include <linux/kmsan.h>
 #include <linux/mm.h>
 #include <linux/mutex.h>
 #include <linux/percpu.h>
@@ -306,6 +307,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	stack->handle.extra = 0;
 	memcpy(stack->entries, entries, flex_array_size(stack, entries, size));
 	pool_offset += required_size;
+	/*
+	 * Let KMSAN know the stored stack record is initialized. This shall
+	 * prevent false positive reports if instrumented code accesses it.
+	 */
+	kmsan_unpoison_memory(stack, required_size);
 
 	return stack;
 }
@@ -465,6 +471,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 	struct stack_record *stack;
 
 	*entries = NULL;
+	/*
+	 * Let KMSAN know *entries is initialized. This shall prevent false
+	 * positive reports if instrumented code accesses it.
+	 */
+	kmsan_unpoison_memory(entries, sizeof(*entries));
+
 	if (!handle)
 		return 0;