[PATCH RFC v4 34/42] kmsan: handle /dev/[u]random

[glider@google.com]

The random number generator may use uninitialized memory, but it may not
return uninitialized values. Unpoison the output buffer in
_extract_crng() to prevent false reports.

Signed-off-by: Alexander Potapenko <glider@google.com>
To: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org

---
This patch was previously known as "kmsan: unpoisoning buffers from
devices etc.", but it turned out to be possible to drop most of the
annotations from that patch, so it only relates to /dev/random now.

Change-Id: Id460e7a86ce564f1357469f53d0c7410ca08f0e9
---
 drivers/char/random.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 909e0c3d82ea..3c69b9772005 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -320,6 +320,7 @@
 #include <linux/fs.h>
 #include <linux/genhd.h>
 #include <linux/interrupt.h>
+#include <linux/kmsan-checks.h>
 #include <linux/mm.h>
 #include <linux/nodemask.h>
 #include <linux/spinlock.h>
@@ -1060,6 +1061,11 @@ static void _extract_crng(struct crng_state *crng,
 	spin_lock_irqsave(&crng->lock, flags);
 	if (arch_get_random_long(&v))
 		crng->state[14] ^= v;
+	/*
+	 * Regardless of where the random data comes from, KMSAN should treat
+	 * it as initialized.
+	 */
+	kmsan_unpoison_shadow(crng->state, sizeof(crng->state));
 	chacha20_block(&crng->state[0], out);
 	if (crng->state[12] == 0)
 		crng->state[13]++;
-- 
2.24.1.735.g03f4e72817-goog